The division that most defines Islam began as a question of leadership rather than doctrine. Muhammad died in 632 without a succession arrangement his followers all accepted. The majority came to hold that the community's elders had chosen rightly in Abu Bakr and the caliphs who followed him; a minority held that authority belonged to Ali, Muhammad's cousin and son-in-law, and to his descendants. Those positions hardened over generations into the Sunni and Shia traditions, and each reads the same early history very differently.
The killing of Ali's son Husayn at Karbala in 680 gave Shia Islam its central act of commemoration and much of its temper. Shia thought developed the doctrine of the imamate: that a line of divinely guided descendants of Ali carried the authority to interpret revelation. The largest Shia branch, the Twelvers, holds that the twelfth of these entered concealment and will return; the Ismailis and the Zaydis count the line differently. Sunnis, the large majority of Muslims, recognise no such office and locate authority in the consensus of the scholars.
Both traditions produced schools of law. Four survive among Sunnis, the Hanafi, Maliki, Shafii and Hanbali, differing on method and detail rather than fundamentals and generally recognising one another as valid; the Shia Jafari school stands alongside them. A Muslim ordinarily follows the school prevailing where he lives.
Cutting across all of it is Sufism, the devotional and mystical current organised into orders around teachers and disciplines of remembrance. For centuries it was the ordinary texture of Muslim piety and a principal agent of the religion's spread in Africa and in South and Southeast Asia. Since the eighteenth century, reform movements arguing for a stripped-down practice drawn directly from Quran and hadith have contested that inheritance, and the argument between them remains one of the live fault lines within Sunni Islam.