just like catholics, orthodox, Protestant, Calvinism...are sects of Christianity, Islam unfortunately has them to. However many Muslims tend to not divide themselves to be any division. Like I'm a Muslim, and that's it. no division or sect.
Sunni Muslims believe that the Mohammad was the last Prophet and there was no more messengers or decedents.
Siya Muslims tend to believe that Mohammad was not the last, in fact there was another prophet, and it was not Mohammad.
Because of each others teachings and strict conservitism and becoming a reactionary, very few (terrorists) believe in killing each other.
A muslim killing a muslim? thats just simply imoral and wrong and a definite sin.
you can find out more about them on wikipedia