Nennies
As you probably know, there's also a division between Sunni and Shia, and there are Sufis as well. While I don't know much about sufism, to be honest, the Sunni/Shia split was, in origin also about a difference in interpretation of Islam. And was enforced through a political conflict after the death of the prophet (saw). The Sunni believed his successor should be elected, the Shia felt it should be his cousin Ali and should be kept within his family.
Shia muslims generally have a different view about certain aspects in Islam. I don't know exactly how they came about, though. Some things even contradict clear evidence from the Qur'an and hadith. (for example Shia beat and harm themselves in remembrance of Hussayn, who was martyred, while Islam clearly prohibits self-harm). I do not know how this came to be, though.
Well first of all I have to say that all sects follow Qura'an and the word of our prophet Mohammad, they all believe in God and they all believe in prophets, angels, etc. the division happens when people try to understand the verses in the Qura'an and the words from Suna, this causing different opinions as Nennies have mentioned before, but it's not like each of them is following a different base or religion or anything.
Now Sunni and Shia, as I recall the whole thing started in the time of the Khalifa Ali bn Abi Talb, he didn't want to be the Khalifa at first but as he became the Khalifa someone didn't like it and said that he should be the Khalifa so Muslims got divided between those who supported Ali and those who supported the other guy and I guess that was the beginning of it, but the whole Shia concept came to action when one of the son's of Al Khalifa Ali got killed (Al Hussien) with other 40 of Al El-Beat after they had been invited by the people of Iraq ( I don't remember which region exactly ) . They were killed in Karbala' and the people that invited them blamed themselves fir what happened, So since then every year on the memory of their death Shia have these kind of tradition to just bring themselves to pain for what they think they had been the reason for happening, they also have some other different stuff in which they also bring to notice the importance of Khalifa Ali and his family to them.
But the important thing is that we are all Muslims, and no one should ever say to someone they are not if they believed in God and said Al-Shahadatean, and the other thing is it's not like there is a person that actually go by one sect, it's like we get what we think better of each, which is not bad, being different should be just for the best of Islam and Muslims.
And if you have any other questions I will be glad to make another quick research and to bring back the pieces of memory I have about the subject.
And may God bring peace upon you.