The religion of Islam has several sects or branches, of whcih the largest denomination is the Sunni (Sunnah) interpretation.
Sunni Islam is based on the belief that the Prophet Muhammad died without appointing a successor to lead the Muslim community (ummah). According to Sunni Muslims, after Muhammad's death, the confusion that ensued from not having a person to head the community led to the election of Abu Bakr, the Prophet's close friend and father-in-law, as the first Caliph. This contrasts with the Shi'a Muslim belief that Muhammad himself appointed his first successor to be Ali ibn Abi Talib as the first Caliph and the first Muslim imam. The sectarian split that occurred in Islam between Sunni and Shi'a Muslims is based upon this early question of leadership.
Thirty years after Muhammad's death, the various factions of the Islamic faith were embroiled in a civil war known as the Fitna. Many of Muhammad's relatives and companions were involved in the power struggle, and the war finally stabilized when Mu'awiyya , the governor of Syria, took control of the Caliphate. This marked the rise of the Umayyad dynasty which ruled Islam until 750. Three sects of Islam developed and emerged at the conclusion of the Fitna: Sunni and Shi'a Islam, and the Khwarij sect, which is generally rejected by Islamic scholars as illegitimate and is today only practiced in Yemen and Oman. Islamic sects that have materialized since the 7th century Fitna, such as The Nation of Islam, are not regarded as legitimate Muslims by Sunni Muslims.
Traditional Islamic law, or Shari'a, is interpreted in four different ways in Sunni Islam. The schools of law, or madhab, developed in the first four centuries of Islam. The four schools of law are the Hanafi, Maliki, Shafi'i, and Hanbali traditions, each based on the beliefs of their founders. Some Sunni Muslims say that one should choose a madhab and then follow all of its rulings. Other Sunnis say that it is acceptable to mix madhabs, to accept one madhab's ruling regarding one issue, and accept another madhab's ruling regarding a different issue. Sunnis also view the hadith, or Islamic oral law, differently than Shi'a Muslims. Hadith are found in several collections, and Sunnis view some of these collections to be more holy and authentic than others, especially the Bukhari collection of hadith.
Even though the main split in Islamic practice is between Sunni and Shi'a Muslims, there are several rifts within the Sunni community. There are some liberal and more secular movements in Sunni Islam that say that Shari'a is interpreted on an individual basis, and that reject any fatwa or religious edict by religious Muslim authority figures. There are also several fundamentalist movements in Sunni Islam, which reject and sometimes even persecute liberal Muslims for attempting to compromise traditional Muslim values. The Muslim Brotherhood and Jamaat-e-Islami organizations are fundamentalist Islamic groups that have given rise to offshoot groups like Hamas who wish to destroy secular Islam and Western society through terrorism to bring back to the world a period of religious Muslim rule.
Some estimates say that Muslims constitute 20 percentof the world's population. Although the exact demographics of the branches of Islam are disputed, most scholars believe that Sunni Muslims comprise 87-90 percent of the world's 1.5 billion Muslims.