The X-Men are Marvel Comics' largest franchise. The general theme of the franchise is learning to cope with bigotry and prejudice while continuing to do what's right.
The X-Men are mutants (humans born with powers, appearances, or abilities beyond those of most people) who are "dedicated to protecting a world that hates and fears them." They were founded by Professor Charles Xavier. His original recruits were Cyclops, Iceman, Beast, Angel, and Marvel Girl. Heroes such as The Mimic, Havok, Polaris, Changeling, and Ka-Zar were introduced in the comics' early issues. Villains such as Magneto, The Brotherhood of Evil Mutants, Blob, Quicksilver, Scarlet Witch, Mastermind, Juggernaut, the Sentinels, Sauron and others were the series' antagonists.
Poorly-written stories, mostly focusing on the X-Men all trying to get into Marvel Girl's pants while thwarting evil caused the series to go into reprints after issue #66, through issue
The franchise was on the verge of cancellation in 1975, when Chris Claremont and Dave Cockrum were allowed to re-vitalize the franchise. They released Giant-Size X-Men #1, which introduced the All-New, All-Different, "International" team of X-Men: Wolverine from Canada, Nightcrawler from Germany, Colossus from the Soviet Union, Storm from Egypt, Banshee from Ireland, Sunfire from Japan, and Thunderbird, a Native American. On the new teams' first mission, Thunderbird was killed in action.
Shortly after, Marvel Girl became Phoenix, when the X-Men ran into the Shi'Ar Empire, and Cyclops met his father, Corsair, leader of the Starjammers. Around this time, the book gained an adjective (as was customary for all Marvel comics), and the team officially became The Uncanny X-Men.
Over the next several years, The X-Men's lineup changed considerably, as member such as Shadowcat, Rogue, Gambit, Jubilee, Longshot, Dazzler, Psylocke, and the original team rotated in and out of the cast. During this time, the original five X-Men formed X-Factor. While the X-Men were thought dead, Professor X formed the New Mutants, who later became the original X-Force. Nightcrawler and Shadowcat ended up in Britain, and formed Excalibur.
In 1991, Jim Lee and Chris Claremont launched a second X-Men title, with no adjective, the first issue of which remains the highest-selling single issue of a comic book in history. X-Factor re-joined the team, and with the cast so huge, the X-Men divided into two "Strike Forces," the Blue Team and the Gold Team. The Blue team consisted of Cyclops, Wolverine, Beast, Gambit, Rogue, Jubilee, and Psylocke. The Gold Team consisted of Storm, Marvel Woman (Marvel Girl all grown up!), Iceman, Archangel (Angel turned dark), Colossus, and newcomer Bishop.
In 1992, Fox Kids began airing an animated series based on X-Men, the team of which consisted of the Blue Team without Psylocke, plus Storm and Jean Grey (who suddenly had no code name). This cartoon, along with the Marvel Universe Series II Trading Cards, is often cited as the reason Psycho Andy got into comics.
The X-Men's roster has grown and shrunk over the years. Currently, in the Marvel Universe, there are only approximately 200 mutants. Following the events of the Utopia/Exodus storyline, the mutant population are living on Asteroid M, which is now an island off the coast of California. Cyclops is seen as the leader of the group, with Emma Frost at his side.