Sunnism itself is a designation for a sectarian group in Islam, so within Sunnism there are no other sects per se. There are, however, different approaches to the application of Islamic law and an Islamic system of belief and practice, called the shariah.
Schisms and Sects
Theological controversies and legal theory were important parts of the early development of Islamic thought. Different schools found ways to accommodate one another and coexist, but some theological divisions resulted in clashes over basic principles of Islamic interpretation.
Missions and Expansion
Islam is a highly adaptive religion, which has absorbed cultural elements from several regions. Hybridization of cultural and religious practices led to varied manifestations of Islam, from medieval Persian court culture in the Abbasid era to Hindu-influenced local religion in Java.
Exploration and Conquest
The most emblematic institution of Sunni authority was the caliphate, the term for the titular head of the Islamic community. Regional and sectarian divisions often came down, throughout Islamic history, to divergent views of who held this central leadership position.
The modern age has seen a series of reform movements in Sunni Islam, from the late 18th century to the present. While historically, some of these have been reactive to the forces of European colonialism or expansion, others have asserted a proactive agenda for Islamic reform.