Shia Islam primarily contrasts with Sunni Islam, whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr (who was appointed Caliph through a Shura, i.e. community consensus) to be the correct Caliph.Unlike the first three Rashidun caliphs, Ali was from the same clan as Muhammad, Banu Hashim.

Shia Islam is based on the Quran and the message of Muhammad attested in hadith, and on hadith taught by their Imams. Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam. The Shia also extend this Imammah doctrine to Muhammad's family, the Ahl al-Bayt ("the people/family of the House"), and some individuals among his descendants, known as Imams, who they believe possess special spiritual and political authority over the community, infallibility and other divinely ordained traits.

The word Shia means follower and is the short form of the historic phrase shiatu ʻAli, meaning "followers of Ali", "faction of Ali", or "party of Ali".Shi'a and Shiism are forms used in English, while Shi'ite or Shiite, as well as Shia, refer to its adherents.

 Terminology
The term for the first time was used at the time of Muhammad.At present, the word refers to the Muslims who believe that the leadership of the community after Muhammad belongs to Ali and his successors. Nawbakhti states that the term Shia refers to a group of Muslims that at the time of Muhammad and after him regarded Ali as the Imam and Caliph. Al-Shahrastani expresses that the term Shia refers to those who believe that Ali is designated as the Heir, Imam and caliph by Muhammad and also Ali's authority never goes out of his descendants. For the Shia, this conviction is implicit in the Quran and history of Islam. Shia scholars emphasize that the notion of authority is linked to the family of the prophets as the verses 3:33,34 shows: "Indeed, Allah chose Adam and Noah and the family of Abraham and the family of 'Imran over the worlds - (33) Descendants, some of them from others. And Allah is Hearing and Knowing. (34)" Shia search for the true meaning of the revelation to get the purpose of the life blood and the human destiny.