The schism between Shiite and Sunni Muslims began almost 1,400 years ago, when disagreements arose over who would succeed the prophet Mohammed as Islam's leader, or caliph.
Though events of centuries ago may seem distant today, many took place in Iraq in locations currently in the news - places such as Karbala and Samarra, the site of Wednesday's bombing of a famous mosque, one of the holiest Shiite sites.
The rift began when the prophet died in A.D. 632. Sunni Muslims, who make up about 85 percent of the world's 1.3-billion Muslims, believe that leadership passed to Abu Bakr, one of Mohammed's trusted companions. Sunni comes from the word sunna, which means the tradition of the prophet.
Shiite Muslims believe Mohammed's direct offspring succeeded him, rather than a caliph selected by a council. They believe Ali ibn Aib Talib, the prophet's son-in-law and first cousin, was the rightful heir. The term Shiite means "advocates for Ali."
Ali was assassinated after he attempted to broker peace between the rival strains.
While all Muslims share some fundamental beliefs about God and Mohammed and the basic obligations of an observant believer, Sunnis and Shiites developed separate traditions in the centuries after Mohammed's death. One of the critical differences is the Shiite belief in a clerical hierarchy.
"The Shia imam has come to be imbued with popelike infallibility, and the Shia religious hierarchy is not dissimilar in structure and religious power to that of the Catholic Church within Christianity," wrote Hussein Abdulwaheed Amin, editor of IslamForToday.com. "Sunni Islam, in contrast, more closely resembles the myriad independent churches of American Protestantism."