The founder of the dynasty Seljuk and his tribe belonged to Oghuz (Oğuz) Turks which can be regarded as the ancestors of Western Turks (today's Turkey, Turkmenistan, Uzbekistan and Azerbaijan). They and their descendants (e.g. the Ottoman Empire) played a major role in medieval ages by setting a barrier against the Mongol onslaught, defending the Islamic world against Crusaders and giving an end to the Byzantine Empire. The fact that they embraced Sunni Islam and defended it with an irresistable power is one of the reasons why this branch is the major and most populous division of Islam.
see Alp Arslan