The X-Men are mutants, a subspecies of humans who are born with superhuman abilities. The X-Men fight for peace and equality between normal humans and mutants in a world where antimutant bigotry is fierce and widespread. They are led by Charles Xavier, also known as Professor X, a powerful mutant telepath who can control and read minds. Their archenemy is Magneto, a powerful mutant with the ability to generate and control magnetic fields. Professor X and Magneto have opposing views and philosophies regarding the relationship between mutants and humans. While Professor X works towards peace and understanding between mutants and humans, Magneto views humans as a threat and believes in taking an aggressive approach against them, though he has found himself working alongside the X-Men from time to time.

Professor X is the founder of Xavier's School for Gifted Youngsters at a location commonly called the X-Mansion, which recruits mutants from around the world. Located in Salem Center in Westchester County, New York, the X-Mansion is the home and training site of the X-Men. The founding five members of the X-Men who appear in The X-Men #1 (September 1963) are Angel (Archangel), Beast, Cyclops, Iceman, and Marvel Girl (Jean Grey); Professor X and Magneto also made their first appearances in The X-Men #1. Since then, dozens of mutants from various countries and diverse backgrounds have held membership as X-Men.
-From Wikipedia.

Lee devised the series title after Marvel publisher Martin Goodman turned down the initial name, "The Mutants," stating that readers would not know what a "mutant" was.

Within the Marvel Universe, the X-Men are widely regarded to have been named after Professor Xavier himself. The original explanation for the name, as provided by Xavier in The X-Men #1 (1963), is that mutants "possess an extra power... one which ordinary humans do not!! That is why I call my students... X-Men, for EX-tra power!"

Many of the X-Men's stories delve into time travel either in the sense of the team traveling through time on a mission, villains traveling through time to alter history, or certain characters traveling from the past or future in order to join the present team. Story arcs and spin-offs that are notable for using this plot device include Days of Future Past, Messiah Complex, All-New X-Men, Messiah War, and Battle of the Atom. Characters who are related to time travel include: Apocalypse, Bishop, Cable, Old Man Logan, Prestige, Hope Summers, Tempus, and Stryfe.

One of the most recurring plot devices used in the X-Men franchise is death and resurrection, mostly in the sense of Jean Grey and her bond with the Phoenix. Though not as iconic as Jean and the Phoenix, many other X-Men characters have died and come back to life on occasion. Death and resurrection has become such a common occurrence in the X-books that the characters have mentioned on numerous occasions that they're not strangers to death or have made comments that death doesn't always have a lasting affect on them. X-Necrosha is a particular story arc that sees Selene temporarily reanimate many of the X-Men's dead allies and enemies in order for her to achieve godhood.

Many of the characters deal with the topic of fate. In particular, Destiny's abilities of precognition have affected certain plot points in the X-Men's history long after she was killed off due to both the X-Men and their enemies constantly searching for her missing diaries that foretell certain futures. The topic of fate takes center stage yet again in a story arc called "The Extremists" involving attacks against the Morlocks due to one of them seeing a dark future for their people. Some characters believe they already know their own fates, such as Apocalypse believing he is fated to rule the mutants or Magneto believing he is fated to lead the mutants to rise up against humans. Other characters such as Jean, Prestige, Evan Sabanur, Hope Summers, and Warren Worthington III have all been wary of their fates and have all taken measures to alter their futures.

Space travel has been a common staple in the X-Men books beginning with the Phoenix and Dark Phoenix sagas. Since then space has been involved in many stories involving the X-Men's allies and occasional rivals the Shi'ar along with stories involving the Phoenix Force. Space has been the setting for many stories involving the likes of The Brood, such as the story arc where the villainous species was first introduced.Through space noteworthy characters like The Starjammers and Vulcan were introduced. Space Travel played a major role in Joss Whedon's run on Astonishing X-Men via the introduction of S.W.O R.D. and especially in one of the final story arcs under his authorship called "Unstoppable" Other notable story arcs involving space included "X-Men: The End," "Rise and Fall of the Shi'ar Empire," "X-Men: Kingbreaker," "War of Kings," and "The Black Vortex."

The topic of sanity has been addressed in many of the major heroes and villains of X-Men. Most famously this is addressed in Jean Grey when she gains near omnipotence through the Phoenix and Professor Xavier after he violently uses his powers against Magneto, unintentionally creating Onslaught. Mystique's sanity wavers throughout the franchise as her constant transformations causes more and more of her mind to fracture.^[32][33] Ever since swapping bodies with Revanche, Psylocke has occasionally struggled to maintain her sanity due to her more aggressive nature and new powers. The character Deadpool is famous for his blatant lack of sanity. After Magneto stripped Wolverine of his metal bones, Wolverine began to become increasingly feral throughout most of the mid to late 1990s X-Men comics. The nature of Rogue's powers affecting her sanity due to her retaining the memories of others has been a central plot device on many occasions, most famously retaining Ms. Marvel's psyche throughout most of the 1980s. Most recently Emma Frost's sanity has become fractured ever since Cyclops died in her arms, causing her to declare war against Inhumans. Other characters who have had issues with sanity include Cyclops, Sabretooth, Magik, Quentin Quire, X-23, and Prestige.

In the Marvel Universe, mutant rights is one of the hot controversial political topics and is something that is addressed numerous times in the X-books as a plot device. While some politicians like Valerie Cooper have legitimately tried to help the X-Men, most have made it their mission to discredit the X-Men in order to eliminate mutants once and for all. Senator Robert Kelly began his platform on a strong outspoken anti-mutant sentiment until he changed his mind after being rescued by mutants later on in his career. When Sabretooth's human son Graydon Creed ran for office, the X-Men sent in Cannonball and Iceman to discreetly join his campaign team and find anything on his anti-mutant agenda. This continued until it boiled to a head when his assassination led to "Operation: Zero Tolerance." Some of the issues presented in the comics serve as allegory to modern issues in the real world, such as Lydia Nance suggesting mass mutant deportation.

By most definitions, characters do not require actual superhuman powers or phenomena to be deemed superheroes. While the Dictionary.com definition of "superhero" is "a figure, especially in a comic strip or cartoon, endowed with superhuman powers and usually portrayed as fighting evil or crime", the longstanding Merriam-Webster dictionary gives the definition as "a fictional hero having extraordinary or superhuman powers; also: an exceptionally skillful or successful person". Terms such as masked crime fighters, costumed adventurers or masked vigilantes are sometimes used to refer to characters such as the Spirit, who may not be explicitly referred to as superheroes but nevertheless share similar traits.

Some superheroes use their powers to counter daily crime while also combating threats against humanity from supervillains, who are their criminal counterparts. Often at least one of these supervillains will be the superhero's archenemy. Some long-running superheroes such as Batman, Spider-Man, Superman, Captain America, Wonder Woman, Iron Man, Thor, the Flash, Wolverine, Green Lantern, and Hulk have a rogues gallery of many villains.
-From Wikapedia

The word 'superhero' dates to at least 1917. Antecedents of the archetype include such folkloric heroes as Robin Hood, who adventured in distinctive clothing. The 1903 play The Scarlet Pimpernel and its spinoffs popularized the idea of a masked avenger and the superhero trope of a secret identity. Shortly afterward, masked and costumed pulp fiction characters such as Zorro (1919), The Shadow (1930) and comic strip heroes, such as the Phantom (1936) began appearing, as did non-costumed characters with super strength, including Patoruzú (1928), the comic-strip character Popeye (1929) and novelist Philip Wylie's protagonist Hugo Danner (1930).

In the 1930s, both trends came together in some of the earliest superpowered costumed heroes such as Japan's Ōgon Bat (visualized in painted panels used by kamishibai oral storytellers in Japan since 1931), Mandrake the Magician (1934), Superman in 1938 and Captain Marvel (1939) at the beginning of the Golden Age of Comic Books. The precise era of the Golden Age of Comic Books is disputed, though most agree that it was started with the launch of Superman in 1938. Superman is the most recognizable Superhero to this day. The success of Superman spawned a whole new genre of characters with secret identities and superhuman powers – the Superhero genre.

1940s

Fox Feature Syndicate's 1930s–1940s superhero the Flame.

During the 1940s there were many superheroes: The Flash, Green Lantern and Blue Beetle debuted in this era. This era saw the debut of first known female superhero, writer-artist Fletcher Hanks's character Fantomah, an ageless ancient Egyptian woman in the modern day who could transform into a skull-faced creature with superpowers to fight evil; she debuted in Fiction House's Jungle Comics #2 (Feb. 1940), credited to the pseudonymous "Barclay Flagg". The Invisible Scarlet O'Neil, a non-costumed character who fought crime and wartime saboteurs using the superpower of invisibility created by Russell Stamm, would debut in the eponymous syndicated newspaper comic strip a few months later on June 3, 1940.

Mr. Scarlet, the "Red Raider of Justice" a superhero appearing in Wow Comics (1940)

One superpowered character was portrayed as an antiheroine, a rarity for its time: the Black Widow, a costumed emissary of Satan who killed evildoers in order to send them to Hell — debuted in Mystic Comics#4 (Aug. 1940), from Timely Comics, the 1940s predecessor of Marvel Comics. Most of the other female costumed crime-fighters during this era lacked superpowers. Notable characters include The Woman in Red, introduced in Standard Comics' Thrilling Comics #2 (March 1940); Lady Luck, debuting in the Sunday-newspaper comic-book insert The Spirit Section June 2, 1940; the comedic character Red Tornado, debuting in All-American Comics #20 (Nov 1940); Miss Fury, debuting in the eponymous comic strip by female cartoonist Tarpé Mills on April 6, 1941; the Phantom Lady, introduced in Quality Comics Police Comics #1 (Aug. 1941); the Black Cat, introduced in Harvey Comics' Pocket Comics #1 (also Aug. 1941); and the Black Canary, introduced in Flash Comics #86 (Aug. 1947) as a supporting character. The most iconic comic book superheroine, who debuted during the Golden Age, is Wonder Woman. Modeled from the myth of the Amazons of Greek mythology, she was created by psychologist William Moulton Marston, with help and inspiration from his wife Elizabeth and their mutual lover Olive Byrne. Wonder Woman's first appearance was in All Star Comics #8 (Dec. 1941), published by All-American Publications, one of two companies that would merge to form DC Comics in 1944.