What is Sufism all about?

Sufism is not something different from Islam. It is all about seeking the the Goodwill and Pleasure of Almighty Allah. In fact, it is putting the teachings of Islam into practice with extreme devotion. The Sufis teach to be kind, helpful and charitable to all human beings irrespective of their caste, creed, color, religion, country etc. They don't hurt the feelings of human beings even if they are sinners. The sinners may repent anytime and become better Muslims and better humans than those who consider them inferior or sinful. All human beings are the off-spring of a single couple - Hazrat Adam (as) and Hazrat Hawa (as). Allah Almighty has created all and He loves all His creatures. Nobody has the right to insult, maltreat or murder anybody. The Sufis welcome anybody who wants to sit in their company. They don't hate any person. They teach to practice tolerance and love. They follow the teachings of Islam in letter and spirit.

Karen Armstrong (Islam: A Short History) says that Sufism is the mysticism of Sunni Islam, but although they had different political views, shared a similar spiritual outlook with early Shi'is. Sufis wanted to get back to the primitive simplicity of the ummah, when all Muslims had lived as equals. The ascetics often wore the kind of coarse woollen garment that was standard among the poor, as the Prophet had done. Where established Islam was becoming less tolerant, seeing the Quran as the only valid scripture and Muhammad's religion as the one true faith, Sufis went back to the spirit of the Quran in their appreciation of other religious traditions. Some saw Jesus as the ideal Sufi, as he had preached a gospel of love. Other maintained that even a pagan who prostrated himself before a stone was worshipping the Truth that existed at the heart of all things.
Shaykh Fadhlalla Haeri (The thoughtful guide to Islam) regards the development of Sufism as natural and progressive, rather than being founded at a particular time or place. By the early eighth century, an increasing number of people had chosen a way of life based on ascetism, meditative and other ritualistic practices, such as chanting the name of God as a means of enhancing remembrance of Allah and maintaining self-awareness. By the ninth century, a large body of teachings from the traditions and Sunnah emphasised the importance of gaining knowledge of the self in order for higher and more subtle knowledge to become accessible.

Ibn Warraq (Why I am not a Muslim) says the earliest Sufis were ascetics, inspired by Christian ideals and seeking salvation by shunning the meretricious delights of this world. Eventually, ascetism was seen as only the first stage of a long journey whose ultimate aim was a deep and intimate knowledge of God. The later Sufis made a complete break with the formal system of Islamic law. Many Sufis were good Muslims, but some were only nominally Muslim, while a third group were "Muslim after a fashion." In the mystic's vision, there were no heavenly rewards and hellish punishments; the written word of God was abrogated by a direct and intimate revelation.