The earliest division within Islam itself happened literally the day after Muhammed died in 632. The well-known division between Sunni and Shi’a Islam separated those Muslims who believed that the keepers of the teaching of Muhammad (the Sunna, or teachings) should lead the movement and those who follow a relative of Muhammad, a son-in-law named Ali.
The party of Ali (Shi’a Ali) became known as the Shi’a, or Shi’ite sect, and today make up only about 15 percent of the Muslim world, primarily in Iran. There is also a presence of Shi’a in Iraq.
The primary difference between Sunni and Shi’a is that the Shi’ites believe Ali was the first rightful caliph. Today, the word imam is used.
These imams are directly descended from Ali himself; they’re considered righteous and to some even infallible. They do not recognize any imam who is outside the bloodline of Ali and they commemorate the martyrdom of Hussein with a festival.
Sunni Muslims constitute the majority of mainstream Islam, from the more nominal Muslim population to some of the most repressive dictatorships in the world. In countries where Sharia is enforced, it is considered to be immutable,and to change it would be to break it – bringing strict consequences.
Sunni Islam typically oppresses its minority Shi’a cousins through persecution, political superiority and in some cases, outright bloodshed, notably in Iraq.
Today, the extreme faction of Islam includes groups such as the Taliban of Afghanistan, the Muslim Brotherhood, the Wahhabi sect of Saudi Arabia, ISIS and Boko Haram.