Basic simplified explanation - the split between the Shiites and the Sunni came about after the Prophet Mohammed died in the 7th Century - there were disputes about who should succeed him as caliph which is a ruler in the Islamic community. The Shia (who make up a small minority of the Muslim world) believed that the Prophet Mohammed divinely chose his son-in-law Ali to be the caliph. The Sunni believed that Abu Bakr, one of Mohammed's fathers-in-law, was the successor. There are other religious intepretations and beliefs that differ, but that's the basic origin of the split.
