The original version of the X-Men was a group of teenagers (never exclusively male, despite the name) who attended Xavier’s School for Gifted Youngsters. By all appearances, the school was nothing more than an elite college preparatory academy in Westchester county, New York. In reality, it served as the training facility and headquarters of the foremost mutant superhero team in the Marvel Universe. Led by the powerful telepath Charles Xavier (also known as Professor X), the first incarnation of the X-Men consisted of Angel, a wealthy playboy who could fly with feathery wings; Beast, a brilliant young scientist whose simian appearance and reflexes belied his intellect; Cyclops, who emitted powerful beams of concussive force from his eyes; Iceman, who could freeze objects and project beams of intense cold; and Marvel Girl (later known as Jean Grey or Phoenix), who possessed the powers of telepathy and psychokinesis.
Mutants like these were both feared and persecuted because of who they were, a theme that resonated in the United States during the civil rights era, and the comic addressed the relationship between the heroic X-Men and a public that did not appreciate, or even want, their help.
In 1975 the series was relaunched with writer Chris Claremont at the helm, and he started a nearly 17-year run that transformed the series from a commercial failure into one of the most influential and lucrative comic books of its era. Claremont introduced a new class of X-Men, giving special emphasis to strong female characters, who he felt were lacking in the industry, and to Wolverine, a brooding antihero who quickly became one of Marvel’s most-recognizable heroes. The characters grew into realistic adults, and the long-running open-ended plots became a template that almost all later X-Men writers followed. By the early 21st century Marvel was publishing a dozen or more X-Men-related comic books each month.
( original text from Brittanica)