Islam, like other religions, has multiple schisms, factions and parties within it. Just like Christianity, there are divergent views. In the Islamic world the largest of these splits is that between Sunni and Shi’ite. Shi’ites represent 15–20 percent of Muslims worldwide, and these are grouped in certain geographic areas, such as Iran, where Shi’ites make up some 90 percent of the Muslim population, and the Gulf countries.
“Sunni” refers to the term “Sunna of the Prophet,” which are the collected sayings and actions of Muhammad–a kind of biography, from which the Islamic community created an orthodox religious textual basis. This, needless to say, suggests that the Sunnis self-identify as “orthodox.” The Sunni heartland is more-or-less the Fertile Crescent, but large numbers also reside in other Islamic countries such as Pakistan where the 80/20 Sunni/Shi’ite split is also present.
“Shi’ite” is the adjectival form of Shi’ism, a term which refers to the Shi’at ‘Ali, or “Party of ‘Ali.”
Shi’ites, therefore, in very basic terms, are Muslims who follow ‘Ali, who was the fourth Caliph (Islamic leader) after the death of the religion’s founder, Muhammad, in 632. ‘Ali had been passed over for leadership three times, but finally became Caliph in 656–only for a brief period, before being assassinated. Followers of ‘Ali believed that the succession should go to descendants of the Prophet through ‘Ali, his cousin and son-in-law, and the Prophet’s daughter Fatima. ‘Ali never managed to generate a unified following during his Caliphate, and the community descended into civil war.
This was the beginning of the Sunni/Shi’ite split, and it initiated the first of several wars within the Islamic community as different factions struggled for leadership.
But what of their differences?
Originally there were few doctrinal differences between Sunni and Shi’ite. The party of ‘Ali comprised, after all, Muslims, and there was only one group of Muslims at the beginning, all following the Prophet. Shi’ism as a political faction took route in areas which are now somewhat peripheral to the “Arab Heartland,” the Fertile Crescent and Arabian Peninsular, with the exception of the Gulf Coast which is today largely Shi’ite. Shi’ite religious practices, therefore, began to diverge from Sunni practices, partly owing to different geographical influences, and in part because of cultural and political circumstances, one of which being the sanctification of both ‘Ali and his son Husayn, who are revered by Shi’ites as martyrs.
The Shi’ite practice of revering the line of the Prophet (not just the Prophet himself), and their reliance on ayatollahs as the reflection of God on earth, are seen by the Sunni community as un-Islamic, as they contradict the strictly monotheistic nature of Islam.
Shi’ism itself splintered again into multiple sects. The largest, with a following mostly in Iran, is “Twelver” Shi’ism, which maintains that there are twelve imams, or leaders, in the line of the Prophet. The last, or twelfth, went into “occultation,” a kind of spiritual hiding, and his authority is vested in the ayatollahs until his “return.” Other Shi’ite branches include the Zaydis who split off after the fifth Imam. They are a mostly Yemeni group. The Ismai’ilis are a mainly south Asian group, although there are diaspora communities around the world, all revering the Agha Khan as the living representative of the Imam.
Shi’ism has had a long history of being in opposition to and even oppressed by Sunni Islam, but apart from representing an alternative vision of Islam, it is heavily influenced by its geographical regions, and the cultural and religious practices prevalent in them before Islam’s arrival. Christians, Jews and Zoroastrians, among others, became absorbed into the body politic of the growing Islamic empire in the eighth, ninth and tenth centuries, and a considerable amount of syncretism occurred, as it did in the wider Sunni community as it spread.
This is useful video explanation: from the Washington Post