Shi'a Muslims reside in all parts of the world, but some countries have a higher concentration of Shi'a. Iran is almost entirely Shi'a, and of the 95% Muslim population of Iraq, about 2/3 are Shi'a. They were oppressed by the Sunni minority under the leadership of Saddam Hussein and the Baath Party.
There are two major groups of believers in Shi'a Islam, with the majority (primarily found in Iran and Iraq) following Twelver version of Shi'a. There are also several forms of Sevener Shi'a sects, the largest being known as Ismailis. The Seveners and Twelvers differ regarding the rights of succession after the death of Muhammad, but they agree that the Sunni have usurped the rightful authority of Muhammad's family descendants.
Also, both Shi'a believe that the last Imam (either the Seventh or the Twelfth) has been hidden alive by God. This hidden (occulted) Imam is capable of sending messages to the faithful. Many Iranian Shi'a believed that the late Ayatollah Khomeini (not to be confused with Ayatollah Khamenei, the current Supreme Ayatollah of Iran) either received messages from the 12th and last Imam, or perhaps even was an incarnation of him.
Beliefs vary as to what will happen when the last Imam, called the Mahdi, or Savior, returns (though some sects reserve that title for Isa/Jesus). It is generally believed that the last Imam will affirm Muhammad's message to mankind from the compassionate and merciful God, Allah.
Members of the Bahá'í Faith also accept the Twelver Shi'a succession as correct, though they also believe that the Twelfth Imám has already returned as the Báb and subsequently Bahá'u'lláh and, as such, adhere to the laws, Scriptures, and institutions of the latter instead of those of Islam.