The X-Men (Xavier's Men), an organization or group of powerful superhuman mutants that was founded and created by Professor Charles Xavier with two main reasons: first, by training mutants how to use and control their powers, and second, to serve as combat team to defend humanity against attacks by powerful mutants aside from them who uses their powers for evil purposes and against other threats.
Their identities (X-Men) are unknown to the public. In their civilian identities the X-Men are just official students or alumni on Professor Xavier's School for Gifted Youngsters. The School, where the X-Men are based, is housed in Charles Xavier's mansion. The Mansion serves as the X-Men's base of operations, training center, and quarters. The true identities of two of the X-Men's founding members, the Angel and the Beast, has become popular in public, they have apparently managed somehow to conceal the fact that they attended Xavier's school from public awareness.
Each member of the X-Men is a superhuman mutant. There have, however, been exceptions, such as the Mimic, who could duplicate the superhuman powers of mutants, and Longshot, an artificially created, genetically engineered humanoid from another dimensional world....
Marvel Universe: The Definitive Source of X-men Informations