Introduction
The word "Sunni and Shi'a appear frequently appear in the Quran. The division of Sunni and Shia started at the death of the Prophet Muhammad, and the question of the successor of the leadership was brought up.
Sunni
The term Sunni comes from the word Sunnah. Sunni is the majority branch of Islam (90%). Sunni Islam responded with the emergence of four popular thoughts on religious jurisprudence. The four schools of law they have are Hanafi, Maliki, Shafi'i and Hanbali. After the death of Muhammad, Sunni Muslims accepted Abu Bakr, one of Muhammad's friends, as their first leader. Sunnis believe that companions (of Muhammad) are the best of the Muslims, because they believe that they were the ones who had the job of compiling the Quran.
Shia
Shi'ite is the the minority branch (10%). Shia Muslims faith is vast and differs from Sunni. Shia Islam embodies a different system that interprets the religion differently, and also has a different political authority. Shia Muslims believe that just as a prophet is appointed by God alone, only God has the prerogative to appoint the successor to his prophet. They believe God chose Ali to be Muhammad's successor, infallible, the first caliph (khalifa, head of state) of Islam. They believe Muhammad, before his death, designated Ali as his successor. Currently, Shia Islam takes up 10% of the population of Iran.