During prehistoric times, the Celestials arrived and used their powers to alter the evolutionary process of several cavemen. As a result, those cavemen evolved into mutants, men and women born with extraordinary powers and/or physical mutations.
Unlike Inhumans and Enhanced Individuals, humans fear and hate mutants. This is the cause for the Mutant Civil War between the X-Men (who act as superheroes to prove to humans that mutants aren't evil and can be trusted) and the Brotherhood of Mutants (who want to kill all humans so that mutants will rule the Earth).
Incredibly powerful mutants are known as Omega Level Mutants.
Some mutants develop "secondary mutations", that give them additional powers or change their appearance drastically. For example, Emma Frost, a mutant with telepathy, developed a secondary mutation that gave her the power to turn her body into a diamond form. Beast, a mutant with super-strength and invulnerability, developed a secondary mutation that gave him claw and fangs and blue fur.