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, despite having long been ruled by the Sunni minority (i.e., until the deposing of Saddam Hussein).
There are two major forms of Shiism, with the majority (primarily found in Iran and Iraq) following twelver Shi'ism. There are also several forms of Sevener Shi'a sects, the largest being known as Ismailis. The Seveners and Twelvers differ on issues of Succesion.
Both believe that the last Imam (either the Seventh or the Twelfth) was hidden by God. This hidden Imam is capable of sending messages to the faithful. Many Iranian Shi'is believed that the 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 was an incarnation of him.
Beliefs upon what will happen when the last Imam returns (usually as the Mahdi, or savior, though some sects accord that title to the return of Isa/Jesus) vary. Most believe that the last Imam will confirm (and perhaps complete) Muhammad's message to mankind.
Members of the Bahá'í Faith also accept the Twelver Shi'a succession as having been valid, though they believe that the Twelfth Imám has 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.