Sunni Muslims comprise the majority of the world's Muslim population. Actual percentages are in dispute, however it is generally agreed that Sunnis make up about 70 to 75% of the total population of Sunnis. The adjective Sunni comes from the word Sunna, which means "habitual practice" and refers to the sayings and actions of Islam's prophet, Muhammad.
The division between Sunnis and Shi'a appeared following the death of Muhammad in 632. The entire community of Muslims agreed that that the leader of the Muslim community included political, as well as religious, leadership. But they did not agree on how Muhammad's successor (or caliph) should be selected.
The split in the Muslim community between Sunnis and Shiites originates in this early moment.
Some believed that the rightful successor was a member of Muhammad's family, Ali, and that future descendents should also be family members. These were Shiites.
Others, who would become known as Sunnis, or followers of the Sunna of the Prophet, believed that the successor did not have to be a family member, but could be chosen by the consensus of designated community leaders, and legitimated through an oath. The first four caliphs were chosen by this process. These four are together known as the Rashidun, or "rightly guided" caliphs, in the Sunni rendering of Islamic history.