X-COM: Apocalypse Strategy Guide/General Tips
There are three sort of agents you can hire.
The standard agent. Most of your staff will be human. They are average agents for all things and a jack of all trades.
Mutants are an interesting twist on human agents. They have more Psi energy, attack, and defense then standard human agents. However they have less health and strength then humans. As a result mutants make good psychers, but fit poorly in other agent roles.
Androids are agents with more health, strength, and accuracy then human agents. In addition to this, they have total immunity to psy attacks. However this comes at a severe cost, Android agents are unable to make psy attacks themselves, nor can they progress. In this sense, they are much like SHIVs in other X-COM games.