Shiite Muslims are the product of the violent schism that followed the death of the prophet Muhammad in AD 632, and through the centuries have generally been the losers in the struggle to lead the faithful and to divide patronage and power. That has left them with a sharp feeling of dispossession, of being a people denied their destiny by corrupt rulers.

The split started with a fight over who should lead the faithful after Muhammad's death. One side believed that direct descendants of the prophet should take up the mantle of caliph – the leader of the world's Muslims. They were known as the Shiat-Ali, or "partisans of Ali," after the prophet's cousin and son-in-law Ali, whom they favored to become caliph. They became known as Shiites.

The other side, Sunnis, thought that any worthy man could lead the faithful, regardless of lineage, and favored Abu Bakr, an early convert to Islam who had married into Muhammad's family. "Sunni" is derived from the Arab word for "followers" and is shorthand for "followers of the prophet."

All religious Shiites nominally observe the advice of an ayatollah on how to follow the law of Islam, or sharia, in the modern context. For many Iraqis, this role is fulfilled by Grand Ayatollah Ali al-Sistani.

The major Shiite holidays celebrate the now glorious defeats and martyrdoms of Imam Ali and Imam Hussein, Ali's son, as typified by the preeminent Shiite holiday of Ashura, which marks the slaughter of Hussein and 72 of his followers by a Sunni caliph outside the Iraqi city of Karbala in AD 680.

In Iraq and Iran, the holiday is marked by elaborate processions of men reenacting their own passion play, many of whom self-flagellate with chains to the beat of drums.

Such expressions of piety are looked at with disgust by hard-line Sunnis like the clergy in Saudi Arabia, who view the veneration of Hussein and other members of the prophet's family as a violation of monotheism. This view has frequently led extremist groups like Al Qaeda to attack Shiites as heretics.

One of the most important distinctions between Shiite and Sunni belief is veneration of the imams.

Most Shiites believe that there were 12 legitimate successors to Muhammad as caliph, and that the final imam, now called the Mahdi, disappeared when he was taken up in the arms of God. Many Shiites believe the Mahdi will return to earth one day and play the role of savior.

The fact that Shiites have long been an oppressed minority – first under the Ottoman Empire, later under states like Iraq and Saudi Arabia – has led to a strong identification with the injustices suffered by Hussein and have lent a political dimension to Shiite worship. Ashura celebrations, for instance, were banned under Saddam Hussein, who feared they could lead to spontaneous uprisings.