X-Man is the name shared by two characters from alternate timelines of the Marvel Universe.
In an alternate timeline (designated Earth-774), the Hulk retained the mind of Bruce Banner. With Professor Charles Xavier and Reed Richards, he formed a conclave devoted to human technological advancement. When Galactus arrived on Earth, the trio used a device called the Psychotron to fuse into one supremely powerful being- the X-Man. Possessed of his components' combined powers, the X-Man was enough of a threat that Galactus decided the effort involved in defeating him would be too great (although not impossible). After Galactus left, the X-Man split into his components, but they were all stripped of their powers.
Little is known about the X-Man's personality except that he had little time for "foolish humans" (and, as a result, accidentally transformed Ben Grimm back into the Thing with some of his stray energies).
X-Man (Nate Grey)
A refugee from an alternate timeline known as the Age of Apocalypse (Earth-295). Nate Grey was created from the combination of the DNA of Scott Summers and Jean Grey, also known as Cyclops and Phoenix, by the evil Mister Sinister, who intended him to be the weapon that would destroy his master, Apocalypse. He escaped after being rapidly aged to teenagerhood, and was befriended by Forge and his group of rogue mutants. Sinister killed Forge, who had been like a father to Grey, and fed the rage that Sinister shaped to make Grey do his bidding.
X-Man was part of the final assault on Apocalypse's citadel, and was preserved to the main timeline after the events that created Earth-295 were undone by the M'Kraan Crystal. Afterwards, he wandered, trying to find his place in this strange new world. After a meeting with the Dark Beast, another refugee from the Age of Apocalypse timeline, Nate learned that his powers were designed to kill him in a few years, as Mr. Sinister had not needed him beyond being an assassin. After this was confirmed by Moira MacTaggart, X-Man began taking steps to restrain his powers, closing off his telekinetic abilities. Despite that, since X-Man was effectively an alternate version of Cable, he still had a formidable amount of psychic and psionic power at his disposal, making him one of the mightiest mutants on Earth.
X-Man recently travelled to an alternate Earth depleted of its life energy by a villain, and sacrificed his own life (and that of the villain) in battle to save that world.