A couple of articles providing a basic overview of the two major sects in Islam and the doctrinal differences between them.
Sunnis and Shias: Islam’s ancient schism
http://www.bbc.com/news/world-middle-east-16047709
Sunnis and Shia in the Middle East
http://www.bbc.com/news/world-middle-east-25434060
IR Theory & Practice 