There are many sects of Islam but these are the 2 pre-dominate ones. You could compare it to Catholics and Protestans, there are many different beliefs in each of these sects.

I cannot really speak much about Shia's as I am a Sunni muslim (I follow the sunna of the Prophet Mohammed PBUH).

Wikipedia has a great deal on this...

http://en.wikipedia.org/wiki/Shi'a

"Diplomacy is the art of saying 'Nice doggie' until you can find a rock - Will Rogers"