The Shia (Arabic: شيعة‎, Shīʿah) represent the largest schismatic sect in Islam, accounting for 10-20% of the world's normative body of Muslims. Adherents of Shia Islam are called Shi'ites or Shias. "Shia" is the short form of the historic phrase Shīʻatu ʻAlī (شيعة علي), meaning "followers", "faction", or "party" of Muhammad's son-in-law and cousin Ali, whom the Shia believe to be Muhammad's successor in the Caliphate.

Shia Islam is based on the Quran and the message of the Islamic prophet Muhammad attested in Shia hadith, and certain books deemed sacred to Shia (Nahj al-Balagha). In contrast to other types, the Shia believe that only God has the right to choose a representative to safeguard Islam, the Quran and sharia. Thus the Shias look to Ali, Muhammad's son-in-law, whom they revere and consider divinely appointed, as the rightful successor to Muhammad, and the first imam. The Shia extended this belief to Muhammad's family, the Ahl al-Bayt ("the People of the House"), and certain individuals among his descendants, known as imams, who they believe possess special spiritual and political authority over the community, infallibility, and other traits.

Although there were many Shia branches throughout history, modern Shia Islam has been divided into three main branches, namely the Ithna ashariyya (Twelvers), the Ismailis (Seveners) and the Zaidis ("Fivers").

Read more about Shia:  Etymology