The Caliphs were the rulers of the Islamic state, both the spiritual head and temporal ruler. Caliph actually means “successor”, and in this instance it means successor to Muhammad the prophet and founder of Islam. Caliphs are given authority for temporal and spiritual matters, but could not exercise power related to religious doctrine. The Abbasids were an Arab family descended from Abbas, the uncle of Muhammad.
They held the Caliphate from 749 until 1258. They united the country in revolt against the Umayyads in 747, under the First Abbasid Caliph.
The Abbasid Caliph moved the capital from Damascus to Baghdad, and Persian influence grew in the empire. The early years were glorious, rising to true splendor under the Fifth Abbasid Caliph, and to intellectual brilliance under the seventh. All of this took place, however, in less than a hundred years. The remaining 400 plus years of the regime were a long, slow, steady decline. In 1258 one remaining family member escaped to Cairo and was recognized as Caliph, and the family survived there, under the Mamluks, until the Ottoman Conquest of Egypt in 1517.