To the outsider, the differences between the Sunni and Shiite Islamic sects are hard to recognize.
The five pillars of Islam – daily prayer; fasting during Ramadan; alms giving; the pilgrimage to Mecca; and belief in one, unitary god – are at the core of both faiths, and most mainstream clerics in each denomination recognize adherents of the other side as "legitimate" Muslims.
The Koran is the sacred text for both. They believe Muhammad was the prophet and that there will be a resurrection followed by a final judgment when the world ends.
Adding to the potential confusion is the insistence of many Muslims not to be identified as Shiite or Sunni, saying they are Muslims and Muslims only.
But, as recent events in Iraq and Lebanon have shown, the differences between the believers are not only seen as important by the communities but now, as they have for centuries, rest at the core of bloody political struggles.
While there are superficial differences between the sects – differences in prayer and carrying out ritual ablutions, for instance – the arena of conflict between the two has long been political.
The split between the two main branches of Islam is nearly 1,400 years old, and started with a fight over who should lead the faithful after the prophet Muhammad's death in 632. One side believed that direct descendants of the prophet should take up the mantle of the caliph – the leader of the world's faithful. 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. In time, they came simply to be known as Shiites.
The other side, the 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."
The Shiites were the eventual losers in a violent struggle for mastery that lasted decades, a fact now reflected in their minority status within global Islam.
But while the civil war now raging between Shiite and Sunni in Iraq is sometimes cast as an extension of this age-old religious struggle, today's conflict is about something slightly different.
While religious differences are real and remain important, the breakdown over Shiite and Sunni in Iraq is about group identity as much as it is about disagreements over proper worship.
In Iraq, many Sunnis and Shiites who are not particularly devout are participating in the bloodshed, fighting to advance group interests.
"I think that Sunni and Shiite group identifiers have become more important in a lot of ways that are not essentially religious,'' says Barbara Petzen, an expert at Harvard University's Middle Eastern Studies Center.
Nevertheless, there are some key religious differences. Shiite veneration of the holy family, that is, the descendants of Muhammad, has contributed to a much more centralized and hierarchical clergy than in the Sunni world.
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 in Iraq, this role is fulfilled by Ayatollah Ali al-Sistani.
Sunni Islam is much less centralized. In this respect, the differences between Sunni and Shiite Islam superficially approach the differences between the Roman Catholic Church and most Protestant denominations.
Though a majority in Iran and Iraq, Shiites make up just 15 percent of the world's Muslims. Their history of defeat and frequent subjugation has also led to a cult of death and martyrdom within Shiism.
The major Shiite holidays celebrate the 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 his followers outside the Iraqi city of Karbala by a Sunni caliph in 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.
The fact that Shiites have long been oppressed – 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.
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. A battle between the forces of good and evil will ensue, ending in a thousand-year reign of peace and the end of the world.
In practice, this leads to occasionally apocalyptic rhetoric from leaders like Iraq's Moqtada al-Sadr and Iranian President Mahmoud Ahmedinejad.
• Staff writer Matt Bradley contributed reporting.