So who are the Shia? The usual way to describe Shi'ism's essence is to say that its adherents have always championed the claim of Ali, the cousin and son-in-law of Muhammad, to be their prophet's true successor.
They believe that rule over the Muslim community must rest solely with Ali's descendants. Shia is indeed a contraction of Shi'at 'Ali - Ali's faction. After Muhammad's death, Muslims who favoured other candidates repeatedly blocked the accession of Ali to the caliphate. When he finally did come to rule, they withheld their allegiance. Later they crushed his family and followers on a desolate plain in modern Iraq in 680. This event, commemorated annually by Shia though the observance of a period of mourning, provided Shia Islam with a deeply emotive drama of martyrdom.
A line of Ali's descendants, the Imams, were persecuted and allegedly martyred for representing a living challenge to tyrannical rule. It is this sense of suffered injustice that came to pervade Shi'ism. The fate of martyrs was all the more poignant as they had been slain by fellow Muslims. To mourn them was also to grieve for the lost of unity of Islam. Even today, Muslim "ecumenism" remains an intellectual exercise, with almost no place in the intimate dialogue between Shia hierarchy and believers. What began as a dissident position on the matter of succession in the seventh century blossomed in time into a full religious tradition, distinguished from Sunni Islam by its own reading of theology and sacred history.
Anthony O'Mahony, "The rise of Shia," The Tablet 7/29/2006