Shia Muslims hold the same fundamentals of faith as Sunni Muslims, with one main exception. They believe true leadership in Islam comes in the form of the Imamate, a quasi-divine institution unique to Shia Islam.
Not to be confused with the Sunni use of the word imam to mean prayer leader, Shia Muslims believe the Imamate represents supreme religious authority as passed through the line of Prophet Muhammad's descendants.
The first Imam was Ali, cousin and son-in-law of the Prophet Muhammad.
His descendants, beginning with sons Hassan and Hussein, continued the line of Imams until the time of the Twelfth Imam, also called Muhammad. The only son of the Eleventh Imam, Muhammad took over on the death of his father in the 10th century, but disappeared without a trace at the age of five.
The Twelfth Imam has since become Islam's only messianic figure. Shia Muslims believe he merely withdrew from public view in what is called "the Great Occultation" and that he will eventually emerge when God commands him to liberate the world from evil.
Referred to as the "Hidden Imam" or the Mahdi [ divinely guided one], Shia Muslims believe the Twelfth Imam remains spiritually present in the world, even if physically hidden. Some believe he appears during certain invocations and prayers.
Iranian president Mahmoud Ahmadinejad often refers publicly to the Twelfth Imam. In September 2005, he concluded his first speech before the UN by calling on God to hasten the return of the Imam. At a theology conference later that year he declared: "The most important task of our revolution is to prepare the way for the return of the Twelfth Imam."
Other differences between Sunni and Shia Islam:
Shia Islam developed into its own sub-sects, including Twelver Shiism, the Ismailis, the Zaidis and the Fatimids. Twelver Shiism, which takes its name from belief in the 12 Imams, is the branch adhered to by a majority of the world's Shia Muslims.
Shia Muslims favour the Hadith [ traditions and sayings of the Prophet Muhammad] narrated by Ali, his wife Fatima and their close associates. Sunni Muslims draw on a wider range of Hadith narrated by Muhammad's companions. This has led to some doctrinal differences, as has the Shia use of ijtihad - independent reasoning - as a means of interpreting religious texts.
Shia Islam allows for the visitation of shrines dedicated to the various Imams and other religious figures. In Iran, these pilgrimage sites include the tomb of the Eighth Imam in Mashhad and that of his sister in Qom. Sunni Islam discourages this practice.
Shia Muslims can combine prayers to pray three times a day, as opposed to the Sunni practice of five times a day. Some Shia pray using a small tablet of clay from a holy place such as Karbala. They place their forehead on the tablet when prostrating in prayer.
The early centuries of Shia Islam, a period marked by political defeat and persecution, led to the practice of taqiya, the dissimulation of one's beliefs. This allows Shia Muslims to hide their true beliefs in the face of persecution.
The practice of mut'ah - temporary marriage - is permitted in Shia Islam. The word mut'ah means pleasure in Arabic. A mut'ah marriage is contracted for an agreed period of time after which it dissolves without need for divorce. Sunnis consider mut'ah marriage haram [ forbidden].