Shia (شيعة), also transliterated Shiah and Shi’ah, is a branch of Islam which holds that the Islamic prophet Muhammad designated Ali ibn-e Abi Talib as his successor and the Imam (leader) after him, most notably at the event of Ghadir Khumm, but was prevented from the caliphate as a result of the incident at Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr, who they claim was appointed Caliph through a Shura, i.e. community consensus in Saqifa, to be the first rightful Caliph after the Prophet.
Twelve Apostolic Imams
The Apostle of Allah said: After me there will be Twelve Imams or Caliphs: and the first of us is Muhammad; the last of us is Muhammad; In between us are Muhammad; and all of us are Muhammad; for all of us are from One Nur (Light).
The Holy Prophet Muhammad (peace be upon him and his progeny) said: "I shall be succeeded by twelve religious leaders, all of whom will be of Qurayshite origin." (Al-Bukhari, al-Sahih)
The twelve Imams are these sacred personages and spiritual leaders about whom the Holy Prophet had prophesied. Pointing them out as the source and means of guidance for mankind, the Holy Prophet remarked: "So long as my twelve successors will continue to govern, this religion (Islam) will exist (in this world)." (Abu Dawud, al-Sunan)
On a request from his well-known companion, Jabir Ibn-e Abdillah al-Ansari, the Holy Prophet explained the names of his twelve successors thus: "They are my twelve successors, O Jabir, who will come after me. First of them will be Ali, who will be followed, one after the other, by Hassan, Hussain, Ali Ibn-e al-Hussain, Muhammad Ibn-e Ali al-Baqer, Ja'far Ibn-e Muhammad al-Sadeq, Musa Ibn-e Ja'far al-Kazem, Ali Ibn-e Musa al-Reza, Muhammad Ibn-e Ali al-Javaad, Ali Ibn-e Muhammad al-Hadi, Hassan Ibn-e Ali al-Askari and lastly by Muhammad al-Mahdi al-Qa'im (peace be on them)."