OK.  There are five major divisions among Muslims, because--just like Jews and Christians did before us--our forebears started squabbling over worldly goods/power once the founders of their respective faiths departed, and claimed they were doing so in the name of 'the true Faith.'  Those divisions are: Ithna-Ashari Shi'a (13%), Zaydi Shi'a (~2.5%) and Maliki, Hanbali, Shafa'i and Hanafi Sunni "sects" which make up the remaining 84.5% of the world's Muslims.

Shi'a are the oldest of the five groups.  The so-called 'salafi' movement, including "ISIS", which was largely armed and financed by the US, are a hateful bunch of murderous ignoramuses--we called their like "Kharijites" in the old days--who take the position that anyone who disagrees with them should be killed.  Starting with the Shi'a.  Being Shi'a myself, I'm kind of against that.  Due to word-count restrictions is difficult to get into the historical details.

Hope this helps a little.