All religions, have denominations and sects. That's normal. Just like countries, 12 tribes of Israel, etc. There is a sense, diversity in the theological phrase: The one and the many. There is also in another sense, the fall of man, that fallen men in sin form absolutes on their own accord without God. The Roman Catholics have sects, the Eastern Orthodox have sects, and not surprising, Buddhism and even Islam, have sects.
Here I am dealing mainly with understanding Muslim Sects.
The best diagram I've found that covers as comprehensive the Islamic sects as possible, is this one, I've also saved this in my drive in case this image gotten moved:
I'll also dive in here into various sects in depths and not just the major two: Sunni & Shia.
Sunni 80% of all muslims = True successor of Muhammad was his father-in-law Abu Bakr.
Shia 20% of all muslims = True successor of Muhammad was his cousin/son-in-law Ali ibn Abi Talib.
Sufi = Mystical form, sort of in between of Sunni & Shia.
Quranists = Qur'an only folks, rejects hadith.
Sunni comes from "Sunnah, the behavior and traditions of the Prophet".
Demographics:
Shafi'i, 29% or the largest of the Sunni subsects. This is in Malaysia.
Most Shia are in Iran, under the Jafari subsect.