Like ghouls, mutants used to be human before radiation altered their DNA. Their bodies range in degrees of mutation. Some are able to pass for human if need be, while others more closely resemble aliens or lab experiments. No two mutants look alike, though they can carry familial traits.
Mutants are better adapted to living in the wasteland, but very few have had access to the same technologies as humans and are unfamiliar with most equipment wielded by human wastelanders. At the outbreak of the End War, after the effects of radiation took hold and started transforming people, those who did not suffer the same fate were afraid and uncomfortable with their irradiated brethren. Those that mutated were not permitted in the vaults and their condition took full affect. The radiation changed the DNA of the affected and over the centuries they have become their own race.
Most mutants hate humans for what they did to the planet. Some are jealous and wish they too were part of the original race, to the point that they attempt to modify their abnormalities and blend in among them. However, most believe they are a more evolved species or that they are superior to humans, and will not pass up the opportunity to try and prove it.