Sunni Islam is the largest branch of Islam, followed by 85–90% of the world's Muslims, and simultaneously the largest religious denomination in the world. Its name comes from the word Sunnah, referring to the tradition of Muhammad. The differences between Sunni and Shia Muslims arose from a disagreement over the succession to Muhammad and subsequently acquired broader political significance, as well as theological and juridical dimensions. According to Sunni traditions, Muhammad left no successor and the participants of the Saqifah event appointed Abu Bakr as the next-in-line. This contrasts with the Shia view, which holds that Muhammad appointed his son-in-law and cousin Ali ibn Abi Talib as his successor.

Islamic schools and branches have different understandings of Islam. There are many different sects or denominations, schools of Islamic jurisprudence, and schools of Islamic theology, or ʿaqīdah (creed). Within Islamic groups themselves there may be differences, such as different orders (tariqa) within Sufism, and within Sunnī Islam different schools of theology and jurisprudence. Groups in Islam may be numerous, or relatively small in size.