Shiites comprise a minority of the world's Muslims. The word Shiite is an Anglicization of the Arabic word Shia, a collective noun that means partisans. It is a shortening of the phrase Shi'at Ali. The partisans of Ali were Muslims in the original seventh century community who believed that the successor to Muhammad, following his death in 632, should be his cousin and son-in-law, Ali.
Ali's partisans believed that Muhammad had overtly designated Ali as his successor, and more broadly that all future community leaders should be Muhammad's direct descendents. Ali's partisans were overruled in the immediate wake of Muhammad's death, and three other caliphs (or, successors) led the community before Ali was designated as caliph in 656.
Challenges to Ali's rule led to two civil wars, Ali's assassination in 661, and the subsequent establishment of Ummayad caliphate. In response, Ali's son Husayn was persuaded to lead a revolt against Ummayad caliph Yazid, in 680. The ensuing battle at Karbala, which is in present day Iraq, led to the massacre of Husayn and his army.
The massacre of Husayn still constitutes the defining moment in Shiite history. It This loss established what Islam scholar John Esposito has called the "paradigm of suffering and protest that has guided and inspired Shii Islam." Husayn is viewed as a martyr in Shiite history, and his death is commemorated yearly on the anniversary of his death.