Sunni and Shi'ite Muslims: what's the difference?

The chief cause of deaths in Iraq is sectarian violence between two schools of Islam, Sunni and Shi'ite. About 65% of Iraq fall in these two groups. 25% are a third group in the north, Kurds, who mostly represent a mild form of Sunnism. The remainder are of various ethnicities.
90% of all Muslims worldwide are Sunni.
Less than 10% of Muslims are Shi'ite.
The two split from a general Muslim faith soon after the prophet Mohammad died in the 7th century.
Sunni follow a series of religious leaders (like Popes) called imams. Iraqi Sunnism is largely in central and northern Iraq. Osama bin Laden is a Sunni Muslim, as was Saddam Hussein.
Shi'ite Muslims, the more orthodox of the two, followed leaders of a bloodline of one of the four leaders who rose after the death of Mohammad (like royal lineage). Shia numbers were very low until the beginning of the 20th century. Shi'ite Muslims are now a majority in Iran, Iraq, and Lebanon, and a minority in Pakistan, India, and other countries. Iraqi Sunnism is largely in southern Iraq. Hezbollah, a Lebanese (terrorist) organisation is Shia.

Sunni and Shi'ite Muslims have not historically been enemies--it is unusual to find any serious conflicts between 700 and 1960--but have engaged in sectarian violence heavily in the last 5-10 years, particularly in occupied Iraq.