Shia (شيعة Shīʿah, from Shīʿatu ʿAlī, “adherent of Ali”), also transliterated Shiah and Shiʿah, is a branch of Islam which holds that the Islamic prophet Muhammad designated Ali ibn 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. Adherents of Shia Islam are called Shias of Ali, Shias or the Shi’a as a collective or Shi’i or Shi’ite individually.
source: al-islam