The X-Men are a team of mutant superheroes founded by professor Charles Xavier in response to both Magneto's terrorist Brotherhood of Mutants, and the U.S. government's Sentinel program.
Mutants are super-powered human beings who were believed by Magneto and Xavier to be Homo Superior, a naturally occuring "next step" in the evolution of mankind (The actual origins of mutants lies in a Canadian laboratory, which attempted to create their own Super-Soldiers via genetic manipulation in response to the United States' World War II deployment of Captain America). Military prisoner James Howlett was the first such "mutant" thus empowered, and it is strongly implied that Magneto's parents were also scientists in this program.
After Xavier and Magneto had a violent falling-out in the Savage Land regarding their differing views on the future of mutant/human relations, Xavier founded the Xavier Institute for Gifted Children in order to train his own students in his integrationist philosophy, and in how to defend humans from mutant aggression and vice versa. The first students were his telepathic/telekinetic protege Jean "Marvel Girl" Grey, and the optic-blaster Scott "Cyclops" Summers, who was trained as the proposed team's field leader. When the Brotherhood and Magneto attacked Washington D.C., the Sentinel initiative went public, causing mutants to be hunted down and summarily executed in the streets. Xavier quickly sent Jean to recruit several young mutants to the cause and give them protective uniforms and appropriate codenames: The superhumanly agile Henry "Beast" McCoy, weather-controller Ororo "Storm" Monroe, and Piotr "Colossus" Rasputin, who could transform into a super-strong steel form. The team's first mission was to rescue Bobby Drake from the mutant-hunting Sentinel robots; Drake was quickly given the codename "Iceman". Howlett, now known as Wolverine and/or Logan, was sent by the Brotherhood to assassinate Xavier, however exposure to both Xavier's teachings and the lovely Jean Grey convinced him to defect to the X-Men. The team also was forcibly drafted into the government's illegal Weapon X hit squad, but eventually escaped.
Later additions to the team included teleporter Nightcrawler, the winged Angel, the foul-mouthed light-manipulater Dazzler, the power-absorbing Rogue, the telepath Psylocke, the intangible Shadowcat, the high-jumping Toad, the pyrokinetic Firestar, speedster Northstar, and double-brained pulse-emitter Syndicate. Others who served or allied with the team briefly include the reality-warper Magician, flame-thrower Pyro, and mass-manipulating time traveler Bishop.
Recently the majority of the X-Men were killed off during the Ultimatum event.