Only the Shiahs believe in the institution of Imaamah. Literally "imaam" means a leader. In Shiah belief an Imaam is the person appointed by God and introduced by the Prophet and then by each preceding Imaam by explicit designation (nass) to lead the Muslim community, interpret and protect the religion and the law (shariah), and guide the community in all affairs.
An Imaam is first and foremost the Representative of God and the successor of the Prophet. He must be sinless and possess divine knowledge of both the exoteric and the esoteric meaning of the verses of the Quraan.
There are many Shiah sects e.g. the Zaidis, the Ismailis etc. The principal sect is the Twelvers (Ithnasharis).
(NOTE: In these Notes, unless specifically stated otherwise, references to the Shiahs and Shiah beliefs, should be construed as references to the Shiah Ithnasheriyya school of thought.)
The Twelvers believe that the Prophet was succeeded by twelve Imaams. These are:
On the death of his father in 260 A.H. the twelfth Imam went into occultation (Gaybah), appearing only to a few leading Shiahs. Until 329 A.H./939 A.D. he performed the functions of the Imaam through representatives appointed by himself. He then went into major occultation which will continue until the day God grants him permission to manifest himself.