The bottom line regarding inter-Islamic fueds is: Everyone hates each other
The origin of the Sunni-Shi’a split is over a disagreement on who should succeed Muhammad and become the next caliph. The Shi’ites believed that Husayn bin Ali, Muhammad’s son-in-law, should succeed him because he was actually related to him (they believe a caliph must be a descendent of Muhammad). The Sunnis argued that since Muhammad didn’t actually specify who was to succeed him, anyone would be fine as long as he was a good Muslim. They chose Abu Bakr, a friend of Muhammad to be the first caliph. Shi’ites don’t recognize the first three caliphs. They believe Ali (the fourth caliph) is the first imam (they use the term imam, not caliph). Shi’a literally means party or sect, from Shi’at Ali (Party of Ali). Sunni comes from sunnah, meaning “tradition.”
Within Sunni and Shi’a Islam there are even more divisions. There are four schools of Sunni thought, with different sects falling under each one. The puritannical Wahhabis (or Salafis as they’re also called) are one of these sects and are actually considered heretics by mainstream Sunnis. Deobandis (Pakistani/Indian Muslims) were originally quite different from Wahhabis but were Wahhabized via Saudi-funded madrasahs, and now the only real theological difference between the two is the school of thought to which they subscribe.
Shi’a Islam is full of divisions. The mainstream Shi’ites (”Twelvers”) are so called because the await the coming of the twelfth imam (like Nutjob in Iran, for instance). “Fivers” recognize only five imams, and believe the fifth one is the “mahdi” (awaited one). The ‘Alawis deviate from mainstream Shi’ism (I’m not exactly sure how, but they don’t believe the 12th imam thing). Bashar al-Assad and the Syrian govt are ‘Alawis. They are generally not looked on favorably by twelvers (except by Nutjob because he’s looking for ME allies). Other sects include Ismailis, Zaidis, Ibadis, and Ahmadiyyahs, with minorites in Yemen mostly. The Druze are an offshoot of Shi’a Islam, but are considered heretics by both Sunni and Shi’a Muslims.
Sunnis hate Shi’ites and treat Shi’a minorites poorly. That’s part of the reason for the Saudi-Iran tensions (the other is the Arab-Persian rivarly). But some branches Sunnis hate other Sunnis, and some branches of Shi’ites hate other Shi’ites. If they didn’t have the Jews to hate to provide a common interest, they would all be tearing at each other’s throats.
Oh yeah, I forgot to mention the Sufis, who are considered heretics by Sunnis AND Shi’ites. The Mevlevi (the “whirling dervishes”) are prominent in Turkey.
And as someone else mentioned, there are also Sevener Shi’ites, who recognize only seven imams.
Sounds like "the Hatfields and the Abdullahs".
Cheers!