Answer 1: Sufism is controversial in Islam. The vast majority of Muslims are either Sunni or Shi'a (or Shia). Some see Sufism as just another Islamic denomination, like Sunni or Shia; but others see it as anywhere from simply a valid way of practicing Islam, all the way to being outside of the Islamic faith tradition altogether. For those who believe that Sufism is not really even a form of Islam, then, obviously, Sufis have made no contribution to Muslim literature.

All Muslims believe that they're on a pathway to a closeness with God in the afterlife, but Sufis believe that it's possible to embrace The Divine Presence in this life, by means of achieving the primordial state of "fitra." Sufis are trained to use "intuitive and emotional faculties" to focus on the more spiritual aspects of religion, and to thereby strive to find divine love and knowledge through the direct personal experience of God by means of a variety of mystical and ascetic techniques and practices. Sufis believe that the angel Gabriel revealed to the Prophet Muhammad a perfection-of-worship practice called "Ihsan," which called upon Muhammad to "[w]orship and serve Allah as you are seeing Him, and while you see Him not, yet truly He sees you."

Some Muslim scholars think of Sufism as a "science" through which one may seek "reparation of the heart and [the] turning [of] it away from all else but God," or "through which one can know how to travel into the presence of the Divine, purify one's inner self from filth, and beautify it with a variety of praiseworthy traits." Sufism, to such scholars, simply describes Islam's inner and more esoteric dimensions... a perfection of worship.

Sufism, then, as defined by its adherents (and the scholars who have written about it), is not so much an Islamic denomination as it is the inner, mystical dimension of Islam in which any Muslim may engage. Therefore, some Sufi orders (called tariqas) consider themselves either Sunni or Shia; while still others claim to be neither, and refer to themselves, intentionally, as distinctly Sufi Muslims. The latter, then, are more likely to either be seen as, or to think of themselves as, more than merely those who practice a more perfect form of Muslim worship, but a Muslim denomination, as well.

Many scholars believe that the basic roots of Sufism are ancient, indeed, and pre-date not only Islam, but also most other modern religions except, perhaps, for Jainism and Buddhism. The word "sufi" is thought to derive from the Greek word "sophia," meaning "wisdom." After Gabriel's revelation of Sufi Ihsan's perfection of worship to Muhammad, the 7th century Muslim Conquests and expansion of the Muslim empire from the Middle East across China and the Indian subcontinent, into Central Asia, across North Africa, Sicily, and the Iberian Peninsula to the Pyrenees mountains caused thousands of adherents and practitioners of other faiths (specifically Christianity, Buddhism and Hinduism) to fall under Muslim control. It is believed (though disputed, especially by those who believe that Sufism falls outside the sphere of true Islam) that during those years Muslim clerics were influenced in their refinement of Sufism by the mysticism and esoterism of Buddhism and Hinduism (and even Christianity), and the example, techniques and methods of Christian hermits and monks who continued their daily faith practices despite their being under Muslim rule. The vast writings, both ancient and modern, about just these early aspects of Sufism have contributed in no small way to the body of Muslim literature.

During the first millennium, several manuals intended to help the Islamic world to understand Sufism were created, the two most famous of which were "Kashf al-Mahjûb" of Hujwiri, and the "Risâla" of Qushayri... both considered important Sufi contributions to Muslim literature. Because some Islamic purists see Sufism as outside Islam (because, as the Salafi sect puts it, Sufism "contains unjustified religious innovation") Sufism's vast writings of simply apologia, which defend and explain it in the face of its critics (and also the works inspired thereby) are also great contributions to Muslim literature. Of course, again, if a given Muslim happens to believe that Sufism is outside of Islam, then s/he will not agree.

Sufism's contribution to other fields of both literature and other endeavor is not insignificant. For example, Sufism's Lataif-e-sitta, or "the centers of subtle cognition" and the awakening of spiritual intuition are thought to have helped to refine such as Hinduism's notion of chakras. Additionally, both Sunni and Shia traditions recognize Sufism's three concepts of nafs, qalb and ruh; and the three resulting spiritual types of tyrant, spiritual moderate, and those lost in love for God. The perfection of Sufi practice stives for the latter.

Sufism has also made no small contributions to general Muslim literature in the area of metaphysics, and the notions of Wahdat al-Wujud (unity of existence), and Wahdat al-Shuhud (unity of witness); or cosmology, based on the Quran's words regarding immaterial beings, the afterlife, the soul, the seven heavens, God, etc. Sufi writings about deeply spiritual approaches to "dhikr" (explained in the next paragraph) are also a huge contributions to Muslim literature.

One of the common practices among Sunni Muslims who call themselves Sufis, is something called "dhikr," which is an Islamic devotional act following prayers in which one typically (and usually silently) recites Islam's 99 Names of God, as well as supplications from the hadith and the Quran. One of the most both beautiful and famous examples of Sufi dhikr is the Sama Ceremony of the Mevlevi Sufi Order in Turkey... also famously known as the "Whirling Dervishes." In 2005, the United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the Mevlevi Sama Ceremony to be one of the world's "Masterpieces of the Oral and Intangible Heritage of Humanity."

If that, and the considerable writings about it, are not a huge Sufi contribution, to not just the Muslim world and literature, but also to the entire world, then it's difficult to imagine what else would be.

Before I answer the question, I would like to briefly discuss what Sufism is.

Sufism is a mode of spirituality within Islam, and using the word "Sufism" connotes something more than the word "Islam" does alone. Sometimes Sufi groups (though usually not traditional Sufi schools and lineages) differ in their approach from the conventional Islam. It is more mystical in its approach, and while the Sufis also have great regard for the Quran and the hadiths, they are commonly thought to lay more emphasis on the parts dealing with virtue and spirituality, rather than fasts, prayers and rituals. In actual fact, most Sufi schools teach a three part path of "Shariah, Tariqah, Haqiqah" each stage of which builds upon the previous as a foundation. Shariah is the religious law and practices of orthodox Islam, which Sufi schools agree must be followed. Tariqah means "path" and is the common way to refer to any Sufi school, and Haqiqah means "truth, reality" meaning God. The message is that by following the accepted Islamic rules and practices, then learning directly from a spiritual teacher within a lineage that has a direct transmission of wisdom through teacher-student relationships all the way back to the Prophet Muhammad, one can walk a path that returns to God.

The word Sufi is believed to have been derived from the word suf which means 'wool', though there are other theories about its etymology.

In India, Sufism is sometimes similar in its approach to the Upanishadic philosophy of the Hindus (the Mughal prince Dara Shikoh from India actually wrote a thesis bringing out their similarities) which encourages the mystic to look for answers himself, particularly by searching within, generally under the guidance of a spiritual master or pir, who is to be treated with utmost reverence, just like a Hindu guru, and so, there is a theory that Sufism emerged as a result of contact with Hindu yogisand traders coming to the Middle East, though Sufis claim that Sufism is no innovation, but derives itself from Prophet Muhammad's teachings, though even they accept that Sufi ideas were influenced by Hinduism, in the early phase of the development of Sufism, due to trade contacts with Hindu scholars, and Hindu mystics and scholars travelling to the Middle East. Sufis also believe that music helps connect man to God, though many conventional Muslims have a rather negative view of music. Sufis also revere scriptures of other religions, and regard that truth is to be found in every religion, and many of them accept Hindu icons like Ram and Krishna to be prophets of Allah, their God Alimghty. A leading Sufi poet, Sinai, clearly stated that people of diverse religions were striving to reach out to the same God Almighty. The Sufis believe in the doctrine of tauhid-i-wajudi or Unity of Being i.e. the same divine spirit being prevalent in all creatures, and that to know God, we need to connect to our inner being, and then, we can become one with God, which is akin to Hindu philosophical doctrines. However, whatever I described above about Sufis is a generalisation, but there are internal differences among the Sufis. The Nakshbandi school of Sufi thought, for example, is more similar to the conventional Islamic view.

Sufi Islam is practised in India, Pakistan, Bangladesh, China, Morocco, Turkey and many other parts of the world. The Uighur people of Xinjiang, who are fighting for independence from China, as they claim the Chinese have colonised them, the way Tibetans do, are followers of Sufi Islam.

The shrines of the Indian Sufi saints are known as durgahs and people of all religions are welcome to pray there. Indeed, a sizeable number of Indian, Pakistani and Bangladeshi Hindus pray in the durgahs in their countries. The most famous durgah in India is that of Khwaja Moinuddin Chishti at Ajmer, which attracts both Hindu and Muslim pilgrims from all over the country and even abroad, and has helped culturally keep India united even when it was politically divided. However, the conventional Muslims consider worshipping Sufi saints as men who became one with God (refer to the doctrine of tauhid-e-wajudi mentioned above), to be sinful, as in their opinion, the Quran makes it clear that none other than Allah, the God Almighty, is to be worshiped.

Hereafter, lies the answer to the question - the impact of Sufism.

While the conventional Muslims may not accept Sufism as the true Islam, Sufism helped form bridges between Muslims and other religious communities and promote a spirit of secular humanism. Hazrat Nizamuddin Auliya, a Sufi saint, whose shrine is located in Delhi, performed yogic exercises and meditation practices so well that Hindus hailed him as siddh (one who has perfected). The Sufis of Kashmir were called Rishis, a term used for Hindu sages. When Sultan Sikander of Kashmir set about pulling down Hindu temples on a large scale, the leading Kashmiri Muslim Sufi, Hazrat Nuruddin Nurani, bitterly protested, arguing that Islam did not sanction this. One eminent Sufi saint from Sind, in what is today Pakistan, even went on a pilgrimage to Hindu holy places. Thus, the Sufis helped cement ties between the Hindus and Muslims of India, at a time when Muslims regarded Hindus to be heretics and were looked down upon, and Hindus regarded Muslims to be untouchables, just like the lower caste Hindus.

The bhakti movement, a movement among the Hindus, which was a revolt against the brahmanical order, criticising the domination of the priestly class and meaningless over-emphasis on ritualistic practices, also reciprocated the same tolerant attitude towards Muslims that the Sufis had exhibited for the Hindus - also an impact of Sufism. Swami Ramanand, a Hindu religious preacher, brought the bhakti movement to North India, which was initially confined to South India, and he was influenced by Sufi ideas. His disciple Kabir was also very strongly influenced by Sufi ideas, and declared that the god of the Hindus and Muslims are one and the same, and drew a large number of follwers, both Hindu and Muslim, from all over North India. It is not clearly known whether Kabir himself came from a Hindu or a Muslim background, but he is as such considered to be a bhakti saint. He is still greatly respected by both Hindus and Muslims all over India.

Nanak, another Hindu preacher and bhakti saint, also drew a large following, comprising both Hindus and Muslims, and he too, like Kabir, preached that the God of the Hindus and the Muslims was one and the same, and hailed the prophets of the Muslims, like Jesus and Muhammad, as well as legendary figures regarded as God incarnate by Hindus, like Ram and Krishna. His order later developed into an independent religion, Sikhism, though many Hindus, who are not officially Sikh, particularly those belonging to the Sindhi and Punjabi communities, both in India and in Pakistan, continue to revere him even today. Kabir and Nanak opposed idol-worship and rejected the concept of incarnation, rather accepting the concept of prophethood, which was in keeping with Islamic ideas. Nanak is held to be the first Sikh guru, and was succeeded by nine others. One of them, Guru Arjan, complied the holy book of the Sikhs, in which he even included verses by Sufi saints like Baba Farid, and bhakti saints like Kabir.

In China, early Muslims, including Sufi teachers were influenced by the language and cultural thought of Confucianism, Buddhism and Taoism, and often expressed Islamic concepts in similar terms. Sufism schools in China are known as Menhuan, while the Islam in China is known as Yisilan Jiao or Hui Jiao. The term Hui Jiao is misleading because it implies that Islam is the teaching (jiao) of the Hui nationality group, the majority of which are Muslim, and that other non-Hui people can not therefore enter the religion. The Sufis in China also helped in maintaining peaceful, healthy relations between people of diverse religions in the country.

The Sufis enriched the literature and music of several languages. Aamir Khusro, an Indian Sufi saint, is credited with the creation of the sitar and tabla, two popular musical instruments in the Indian subcontinent, and he is also regarded as the first poet of the Hindi language, which is today the national language of India. The Sufi tradition of kawali or group singing in a certain fashion influenced the Hindustani school of classical music of the Indian subcontinent. An acclaimed Sufi musician of the contemporary era is the late Nusrat Fateh Ali Khan from Pakistan. Today, Sufi musicians in Pakistan have produced a new genre of music, known as Sufi pop music, which is popular not only in Pakistan, but even in India. In fact, this genre of music has expanded itself into the non-religious realm as well in a very big way.

An internationally acclaimed Sufi from Turkey is the late poet Rumi.

Sufism also was a step forward in the direction of women's empowerment. While as per Islamic belief, Prophet Muhammad said that women should not be stopped from going to the mosque if they so desire, many mosques in the Indian subcontinent barred them, and still bar them entry, but there is no such restriction in a durgah or Sufi shrine, where men and women worship alongside each other. In fact, some of the Sufi saints, such as Rabia from Kashmir, were women.

While the Sufi saints in the Indian subcontinent were mostly not concerned with conversions, the popularity of the Sufi movement is believed to have been one of the factors for Islam emerging as a religion with a large following in what is today India, Pakistan and Bangladesh, though the hindutva scholars and historians argue that Islam was basically spread in India mainly by force, a theory that most mainstream historians do not accept, though that there were some forcible conversions to Islam at some points of time in Indian history is a generally accepted fact. Also, jiziya, a tax for the non-Muslim members of the state (as opposed to the zakat tax for Muslim members), was higher, and this was also likely a factor that contributed to the spread of Islam in India, so the Sufi movement alone cannot be ascribed as the reason for the rise of Islam in the Indian subcontinent.

However, many of the rulers like Akbar and Muhammad bin Tughlaq were strongly influenced by Sufi ideas and ruled the State in a secular fashion, respecting the rights of non-Muslims at par with the Muslims. They abolished the jaziya and gave non-Muslims high posts in the government administration and even representation in the royal court. While both faced opposition, Muhammad bin Tughlaq had to pay for his secular policies more dearly than Akbar.

Akbar even started his own order of the Sufistic type, called tauhid-e-ilahi or 'unity of God' wherein after studying different religions, he felt all religions carried truth in them, and that the best parts of them should be integrated. He was not, as is often stated, creating a new religion. His order tauhid-e-ilahi appears to have been referred to as din-e-ilahi, din meaning religion, the first time more than half a century after his death. Joining his order did not mean giving up one's religious identity, just as many Hindus venerated and still venerate the Sufi saints, and still remain Hindus. Thus, Akbar wanted Hindus, Muslims, Christians, Zoroashtrians and others to voluntarily join his order, accepting that the god of all these religions is the one and the same, and follow his teachings, accepting him as their spiritual guide, but his order was not meant to be a new formal religion to which they had to convert as such. However, one of Akbar's most prominent critics was Shaikh Ahmad Sirhindi, who belonged to the Nakshbandi school of Sufism, which as mentioned above, is similar to mainstream Islam. There were also many other rulers, other than Muhammad bin Tughlaq and Akbar, who were strongly influenced by Sufi ideas.

Thus, this has been the impact of Sufism. However, another impact has also been intense intellectual and violent conflict within the Islamic world, which some scholars like Al Ghazzali tried to bridge by stating that Sufism is alright as long as it does not overlook or violate any Quranic principles. Occasionally in Islamic history, Sufis have been persecuted by rulers who suspected them of spreading an unacceptable degradation of the religion. A significant example can be the killing of the popular Sufi saint Mansoor. Also, minority Muslim groups such as Wahabis/Salfis (a conservative movement started in Saudi Arabia by Muhammad ibn 'Abd al Wahab in the mid 1700s) often oppose Sufi doctrines as being un-Islamic or even being contradictory to Islam.

However, it would be incorrect to say that only the followers of Sufi Islam are liberal and moderate Muslims. In fact, apart from the few fanatics that will appear in any group of people, ALL Muslims are opposed to killing innocent people, which is definitively against Islam, in letter and spirit. To them, tolerance means 'live and let live' but not accepting other religions as necessarily right in their own ways. Also, many of the conventional Muslims respect the Sufi saints as great men in history, though they may have differences of opinion regarding their teachings or practices.

Some scholars like William Darlymple seem to suggest that followers of Sufi Islam are liberal and tolerant, while the negative stereotypes about Muslims are somewhat valid for the conventional Muslims. This is a gross generalisation about the latter, and even about the followers of Sufi Islam, for some schools of Sufi thought, like the Nakshbandi school, resemble the conventional Islamic view. Also, some try to paint a picture that while Sufism is a movement of Muslims, it has little to do with the Quran or hadiths, something most followers of Sufi Islam strongly refute, and rightly so, for many of the verses written by Sufi poets are based on the life and teachings of Prophet Muhammad and other prophets of Islam, like Moses and Jesus.

* I am aware that my answer deals primarily with the Sufi Movement in the Indian subcontinent only, though I have mentioned that Sufism is prevalent in many other parts of the world also, like Morocco, China and Turkey, and I even mentioned the name of Rumi, a Turkish Sufi poet, and a little about Sufism in China. The reason for this Indian subcontinent-centric approach is that I am an Indian myself, and so my knowledge is more Indian subcontinent-oriented. This apart, I think Sufism perhaps impacted the Indian subcontinent more than elsewhere, because here, Islam has existed in a climate of religious pluralism more than Muslim-majority countries, and the Sufis are venerated here even by non-Muslims which makes their impact more significant. Also, many Sufi ideas are akin to Hindu ideas, and Hinduism is the majority religion of India, and a prominent religion in Pakistan and Bangladesh.