As the unrest in the Middle East continues, a lot of focus on the street is on the divisions between Sunni and Shiite followers of Islam. Here is a brief description of the differences and history of the two branches.
Sunni is the largest and more orthodox branch of Islam. Sunni's believe that the only rightful heirs to leadership are the heirs of the first four caliphates ("dominion of a successor") - Mohammed's successors. Approximately 80% of Muslims are Sunni. Shiites believe only heirs of the fourth caliphate, Ali, are legitimate leaders.
Shiites believe the Mahdi, the rightfully guided ruler whose role is to bring a just caliphate to Earth has already emerged in history, while Sunnis believe he is yet to come. Shiites believe the Ayatollah Khomeini is the Mahdi and are most commonly found in Iran, Iraq and Lebanon.
The loss of the caliphates after WWI devastated the Sunni branch of Islam, and caliphates modeled on Western principles in Egypt and India were rejected wholesale. Osama Bin Laden is a Sunni, and the Muslim Brotherhood stemmed from the unrest and upheaval caused by the loss of caliphates in the 1920s.
Sunni/Shiite discord is recent, only dating back to the 1960s and largely stems from Western backing of Shiite leaders who were brought to power during this time.