Islam consists of a number of religious denominations that are essentially similar in belief but which have significant theological and legal differences. According to most sources, approximately 85% of the world's Muslims are Sunni and approximately 15% are Shi'a, with a small minority who are members of other Islamic sects.
Sunni Muslims are the largest group in Islam. The Sunnah (the example of Muhammad's life) as recorded in the Qur'an and the hadith is the main pillar of Sunni doctrine. Sunnis believe that the first four caliphs were the rightful successors to Muhammad; since God did not specify any particular leaders to succeed him, those leaders had to be elected. Sunnis recognize four major legal traditions, or madhhabs: Hanafi, Maliki, Shafi'i, and Hanbali. There are also several orthodox theological or philosophical traditions within Sunnism.
The Shi'a, who constitute the second-largest branch of Islam, believe in the political and religious leadership of Imams from the progeny of Ali ibn Abi Talib, who according to most Shi'a are in a state of ismah, meaning infallibility. They believe that Ali ibn Abi Talib, as the cousin and son-in-law of Muhammad, was his rightful successor, and they call him the first Imam (leader), rejecting the legitimacy of the previous Muslim caliphs. To most Shi'a, an Imam rules by right of divine appointment and holds "absolute spiritual authority" among Muslims, having final say in matters of doctrine and revelation. Shi'a Islam has several branches, the largest of which is the Twelvers which the label Shi'a generally refers to. The Twelver Shi'a follow a legal tradition called Ja'fari jurisprudence. Other smaller groups include the Ismaili and Zaidi, who differ from Twelvers in both their line of successors and theological beliefs.
Sufism is a mystical-ascetic form of Islam. Sufis strive to obtain direct experience of God by making use of "intuitive and emotional faculties" that one must be trained to use. Sufism has been criticized by some Muslims for being an unjustified religious innovation. Most Sufi orders, or tariqas, can be classified as either Sunni or Shi'a.
The Kharijites are a sect that dates back to the early days of Islam. The only surviving branch of the Kharijites is Ibadism. Unlike most Kharijite groups, Ibadism does not regard sinful Muslims as unbelievers. The Imamate is an important topic in Ibadi legal literature, which stipulates that the leader should be chosen solely on the basis of his knowledge and piety, and is to be deposed if he acts unjustly. Most Ibadi Muslims live in Oman.