According to the Twelve-Imam Shiite dogma, imam refers only to one of twelve early leaders of Islam - Ali, Hasan[?], Husayn[?], and nine of Husayn's descendants.
The Ismailis trace a different line of Imams, branching at one of Husayn's descendants from the 12-Imam line.
For a slightly more detailed description of the Shiite belief (and for the other names and titles of the twelve Imáms, see, for example, http://www.bahai-library.org/books/dawnbreakers/preface/prefislam.html .
wikipedia.org dumped 2003-03-17 with terodump