I got this from the Christian Research Journal.
Most of the difference centers upon the disagreement over who should succeed Muhammad, who died June 8, 632 AD without any sons. Four “Rightly Guided” caliphs followed:
Abu Bakr (632-634 AD)
Umar (634-644)
Uthman (644-656)
Ali (656-661), a cousin of Muhammad
Sunni/Sunnite Islam: Sunni Islam argues that the true successors to Muhammad do not need to be descended from the immediate family of Muhammad. Sunni Islam emphasizes tradition in religious life, the way the Prophet and his followers lived. Thus, the Quran is the fundamental authority, along with the Hadith and the consensus of the community. Saudi Arabia, including Mecca, is the center of power for Sunni Islam.
Shia/Shiite Islam: Shia Muslim argues that the true successors to Muhammad descend directly from Muhammad’s family. Ali (Muhammad’s cousin) and his sons were therefore the rightful heirs to the leadership of Islam, but they were killed by those distorting and misusing power in the decades after Muhammad’s death. Even though they are a minority, Shia Muslims believe that a spiritual power of their true successor is transmitted to a series of Imams. Although there is disagreement within Shiism on the number of Imams and their exact progression, most Shiites assume that the spiritual power of the Imams works through and guides Shiite leaders. They assume that the last Imam will return to restore true Islam and inaugurate a new age in world history. Most Shiites are found in Iran, Pakistan and Iraq. They are only 10 percent of the Muslim world and comprise half of the Muslims in the Middle East. They pray only three times a day, have a passion for martyrdom, and practice muta (temporary marriages).
How important is this difference? Saddam Hussein was a Sunni. Since Iran is mostly Shiite, Iran and Iraq have always been at odds. Hussein lied about weapons of mass destruction because he didn’t want to appear weak to his enemy Iran, who are Shiite. The war there today had its roots in this conflict that dates back 1,400 years.
