Answer:
Prophet Muhammad did not leave a successor after his death, leaving his community to chose between the best of his companions. The first caliph was Abu Bakr, the Prophet's best friend. Then Omar, Othman and Ali followed. During the caliphates of Omar and Othman there were groups that stated that the successor of the Prophet should have been Ali, as he was from the Prophet's family and he married his daughter Fatima. After the death of Othman, the community had two leaders at the same time: Ali and Muawiyah, the latter claiming Ali was not fit to rule the entire Empire. After these moments, two different sects emerged: the sunnis, who consider that Abu Bakr was the rightfull successor, and the shiites, who consider that Ali should have been and that only from the Prophet's family should be chosen a leader. The religious beliefs of the two sects have common aspect, but also differencies. Nowadays there are aproximately 70% sunnis and 30% shiites, but the statistics are very vague. The largest number of shiites live in Iran, Iraq, Liban, Syria and Yemen.