The X-Men are a team of comic book superheroes from the Marvel Universe. The team members are mutants, each inborn with an ability that goes beyond what is humanly possible. In this way--being human but possessing abilities that go beyond the possible--mutants are unusual. Such mutants are subject to discrimination in the Marvel Universe, and as such must keep their status secret, but the founder of the team believes that peaceful coexistence is possible, and that mutants can use their powers to help all of humanity. Notably, their first mission was fighting Magneto, a mutant supremacist who believed that normal humanity's time had come and gone.
The initial members were Charles Xavier (Professor X), a powerful telepath, who founded and funded the initial team under the guise of it being a "school for gifted youngsters"; Scott Summers (Cyclops), who can project bolts of force from his eyes; Henry McCoy (Beast), an immensely strong, agile, tough, and athletic man; Robert Drake (Iceman), able to generate cold from his body; Warren Worthington III (Angel), possessed of feathered wings and with different enough physiology to let him fly using them; and Jean Grey (Marvel Girl), who possesses telekinetic ability. Perhaps the most notable member of the team is not a student at the school; it is Wolverine (Logan), a mutant with enhanced strength, speed, and instincts, known for flashing claws made from a near-indestructible adamantium skeleton when he is angry. Many stories within the comics explore Wolverine's history and psychology.
Since the comic's first issue in 1963, the roster of the team has changed. Characters have left, joined up, changed their code names, switched sides, died, been resurrected, and developed new powers or new ways to use them. The current roster actually consists of several teams, as many more mutants have been found all over the world and many have joined or created teams with deep ties to the X-Men.
Being created in the '60s, the comic was deliberately written to be a metaphor for the social issues of the time, such as discrimination, specifically racism (despite the entire team being composed of caucasians). Since then, the themes of the various titles with the "X" brand have been expanded to be a metaphor for tolerance and acceptance in general--for other races, for other modes of belief, and other sexual orientations.
While the books' premise does not require evolution to be factual -- mutations are known to occur -- the premise is explored in a way that assumes it is. A storyline in the early 2000s suggested that "regular" humanity, homo sapiens sapiens, would be extinct within a century, replaced entirely with the mutant species homo sapiens superior. Since then, a new event decimated the mutant population and prevented the mutant gene--the "x-factor"--from being passed on to new children, meaning it would be mutants going extinct within a generation, but this is unlikely to last, as some of the effects of the event are already "wearing off" or have otherwise been rectified.
The X-Men franchise spawned a series of movies in the 2000s and is still popular today.