An excellent article from the Get Religion blog that sheds a bit of light on the doctrinal differences between Shiite and Sunni Muslims: Meanwhile, back at the ranch (Yeah, it's a stupid title, but still a good post.)
It is CRITICAL for one to understand the Muslim religion in order to understand the motives of those who pursue terrorism. About 99% of the justification for terrorism comes from a very conservative reading of the Koran. The other 1% is disgust over Brad Pitt leaving Jennifer Aniston for Angelina Jolie. Really, what the heck was he thinking?