What is the Sunni-Shiite division?

Answer 1

The initial split between the Shiites and Sunnis came when their prophet, Mohammed, died in 632. At that time, the Muslims controlled most of what we now call the Arabian Peninsula; the question came to be: who should be the new leader now that the prophet was gone? The Shiites and Sunnis disagreed over who the first new leader, called the Caliph ("successor"), should be, and how he should be chosen.

• Sunnis believed that the Caliph should be chosen by the shura, which is the consensus of the Ummah(community). Basically, the various leaders would get together and choose who they think is the best person to be Caliph.
• Shiites believed that the first Caliph was chosen by Mohammed himself before his death. Additionally, Shiites believe that only an imam (someone descended from Mohammed's daughter Fatima) could be the leader.

The first Caliph was Mohammed's father-in-law Abu Bakr, who was chosen using the Sunni method. Shiites believed that Muhammed had nominated Fatima's husband Ali to be the first Caliph. Both sides could point to verses and quotations to support their point of view, so it wasn't easy to figure out who was right- the two sides still disagree.

So, to shorten a long and complicated story, eventually Ali did indeed become Caliph (the fourth one, after Abu Bakr, Umar and Uthman, all having had fairly short reigns). Ali also had a short reign (about five years, and then he was assassinated), during which a civil war between the two factions was fought and this solidified the schism. The two factions grew apart, and have since developed different traditions, customs and beliefs on how their religions should function.