Seljuks are originally Turkish tribespeople, who migrated into western Asia in the 10th century while fighting with various tribes on their way. They accepted Sunni Islam and founded dynasties in Persia, Mesopotamia, Syria and Anatolia. The Seljuk Empire's lands covered approximately today's Kyrgyzstan, Uzbekistan, Turkmenistan, Kazakhstan, Turkey, all of the Middle East and a part of the Arabian peninsula.
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 (i.e., the Ottoman Empire) played a major role in medieval times by setting a barrier against the Mongol onslaught, defending the Islamic world against Crusaders and bringing 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