Sorry I don't know much but I can tell you this. :)
There used to be only one sect of Islam. People called Muslims. But after Muhammad died. Muslims had argued who should be their new leader. Muslims separated into 2 different sects. Sunni Muslims and Shiite Muslims. Shiite Muslims believe Muhammad's cousin and son-in-law will lead the Shiites. Sunni Muslims believe Abu Bakr, which was the prophets advisor, to lead the Sunnis. The most popular sect of those 2 is Sunni Muslims.