Mutants are terrestrial creatures that derive their powers from their genetic code. In many cases the mutation gene can be passed down through reproduction, though some types of parasitic mutations, such as lycanthropy, zombification, and vampirism, can be passed on to a living host. Mutants are a diverse group, with many different powers, abilities, and appearances. Mutants are almost always distinguishable from normals physically as well as genetically.
There are several external works that are excellent references on mutants.
Subject 42 -part of the back story for Sheppard. It goes more into depth on the nature of mutations and identifies how parasitic mutations differ.