Short answers, pulled from the story.
The first X-Men comic book was published in September 1963. The title The X-Men #1 hit newsstands in that month, introducing a group of teenagers with powers they could not control and a teacher who could read their minds.
Stan Lee and Jack Kirby created the original X-Men team in 1963. They developed a group of mutants born with an X-gene that granted them superhuman abilities, usually manifesting during puberty.
The new international X-Men team launched in May 1975. A new team of mutants was introduced in Giant-Size X-Men #1, marking a turning point for the franchise with characters from diverse countries.
The first issue of the second X-Men series, launched in October 1991, holds the record for highest sales. It sold more than eight million copies, making it the best-selling comic book of all time according to the Guinness Book of World Records.
Marvel announced they would cancel all X-Men titles and relaunch the entire lineup on the 14th of May 2019. Jonathan Hickman was given full creative control, beginning with two rotating bi-weekly six-issue limited series called House of X and Powers of X.
The X-Men comic series allegorizes real-world conflicts experienced by minority groups in America such as African Americans, various religious or non-religious groups, ethnic minorities, autistic individuals, and the LGBTQ community. The story explores themes of prejudice, racism, antisemitism, and civil rights through the mutant metaphor.