The X-Men is a group of guys that banded together because nobody else wanted them. The X-Men is composed of mutants, a type of genetic freak that is both dangerous and uncontrollable. The X-Men is one of the largest teams in the world, after the Avengers.
The X-Men were founded by Professor X when he realized that mutants were stupid by nature, and he could make them smart. He tried to make them smart, but he failed. Then he tried to make them intelligent, but he failed at that too. Then he tried to make them smart. He failed at that. That's when Cyclops slapped him for being an idiot. After that they turned their focus to weaponizing small children.