Sunni Muslims make up most of the muslim population in general, the Shi'a is a small percentage. The Sunni's believe the following of Muhammad's close friend after he died, while the Shi'a believe in the following of the blood line, not a friend.
They have 5 pillars they must do to be considered a Muslim: accept Muhammad as their profit, pay money to the needy, pray 5 times a day facing Mecca, fast during Ramadhan (no eating between sun up and sun down), and then going to Mecca once in their lifetime.
I'm not muslim myself, but half my family is. So I know quite a bit about it without actually being apart of it myself (I don't eat pork though).