The original split between Sunni and Shi happened shortly after the death of the Prophet Muhammad. The Sunni people are considered as the orthodox branch of Islam, traditionally led by Ahl-al-Sunnah, which means people of tradition. They are guided by tradition and put it first. Shia emphasize learning by the successors of Muhammad.
After the first caliphates, after Muhammad's death, there was a dispute over who would manage caliphate, be caliph. The Caliphates were centers of political and religious power. The Sunnis had a stance, guided by the tradition, that any Muslim who proved to be capable could be a caliph, while Shia claimed that only the successors of Muhammad can be caliphs, those who are related to him.
After Muhammad's death, Muslims needed someone to rule according to their holy book, the Qu'ran. It divided the Muslims because of their disagreement on who they thought would be an ideal leader (caliph). Sunni's believed that any Muslim most capable of ruling should be able to rule, but the Shia believed that a Muslim leader would need to be a descendant from Muhammad and related in some way.
