There are two main traditions in Islam: Sunni and Shi'ah. The word 'Sunni' is related to the Arabic word Sunnah and means 'followers of the Prophet's Sunnah or tradition'. Sunnis make up the majority of Muslims worldwide. The word 'Shi'ah' comes from a phrase meaning 'followers of Ali'. Ali was the Prophet's son-in-law and cousin.
Historically, the different traditions come from a difference of opinion over who was to lead the Muslim community after the Prophet Muhammad's death. Sunni Muslims believed the Prophet's successor should be his father-in-law and disciple, Abu Bakr. Shi'ah Muslims supported the Prophet's blood relative, Ali, as his true successor.
Despite these historical differences, the two groups share many beliefs. For example, both groups believe in the oneness and unity of Allah, the importance of the Prophet Muhammad and his Sunnah, the Five Pillars of Islam and the Qur'an.