Shia Islam is the second largest denomination of Islam, after Sunni Islam. The followers of Shia Islam are called Shi'ites or Shias. "Shia" is the short form of the historic phrase Shi'atu Ali, meaning "followers of Ali", "faction of Ali", or "party of Ali".
Similar to other schools of thought in Islam, Shia Islam is based on the teachings of the Islamic holy book,the Qur'an and the message of the final prophet of Islam, Muhammad. In contrast to other schools of thought, Shia Islam holds that Muhammad's family, the Ahl al-Bayt ("the People of the House"), and certain individuals among his descendants, who are known as Imams, have special spiritual and political authority over the community. Shia Muslims further believe that Ali, Muhammad's cousin and son-in-law, was the first of these Imams and was the rightful successor to Muhammad and thus reject the legitimacy of the first three caliphs.