Affected: The villains of Apocalyptus. Horrible mutants. They are mostly human, and their abilities are usually defined by their profession or social standing. For the most part, they're dumb, and clever plans to outsmart them are usually effective. They rule the surface of London, and much of the rest of the world.
Trueborn: The heroes of Apocalyptus. They have as much radiation in them as the Affected, but for unknown reasons, it doesn't make them mutate. It just gives them cool powers.
Energy Points: Energy points represent radioactive power held by characters, which allows them to perform spells and stuff. Every player class has abilities that use energy points. Non-spellcasters use them to do extra-cool moves in combat.
Faith: Every character has a faith score. Faith grows with level, and can be attacked like HP. Characters whose faith is depleted cannot use any Trueborn powers.
PSI: PSI represents a character's innate ability to use steam-powered machines. Steam mages have it naturally. Others can get limited amounts by purchasing boilers.
Taint: Sometimes a Trueborn character will come into contact with especially evil radiation, or may even use a technique that causes him to generate especially evil radiation. This radiation is called taint, and is accumulated in a characters taint score. Every time a character's taint reaches a new multiple of ten, he gains a permanent mutation. These mutations are never beneficial.
Dragoon: A class very similar to the Fighter from Dungeons and Dragons. In addition to regular D20 feats, he gets Combat Maneauvres (intentionally misspelled in a parody of British spelling) which allow him to do cool moves by spending energy points.
Anglican: The healer class. In the Apocalyptus world, pretty much all religions have merged into Anglicanism. Depending on what religion a given Anglican follows, he may have very different powers.
Steam Mage: Steam Mages are powerful machine-users, and have the ability to generate "steam fields" around themselves, which are passive areas of effect.
Scoundrel: An extremely skill-focused class. Scoundrels are proficient in all skills, and can choose which ones they want to use as enhanced skills. Enhanced skills allow them to use skill checks to perform actions that range from unusual applications of their skills, skill-based combat abilities, and even effects similar to spells.
Nukeromancer: The most powerful caster class. Nukeromancers get incredible destructive ability at the expense of taking tons of taint points. A high-level Nukeromancer will be able to kill most anything, but will be crippled and insane due to his many mutations.
Persuasionary: Persuasionaries specialize in mind-control abilities. They are accompanied by companions, who can aid them in a lot of interesting ways.
Gunpowder Wizard: A class from my World's Fair expansion currently in very early playtesting. Gunpowder Wizards create clouds of gunpowder. Whenever they attack an enemy inside one of their gunpowder clouds, they get a huge damage bonus like a rogue's sneak attack.
Grenadier: Another World's Fair class. Grenadiers get the same moves as Dragoons, but less of them. They compensate by getting way more hit points than is reasonable.
Mad Scientist: Another World's Fair class. Mad Scientists get multiple frankenstein-style monsters to control. They have some of the best buff spells in the game, but can only use them on their own creations.
Occult Detective: Another World's Fair class. Specializes in fighting paranormal threats. This one needs a lot of work right now, as it is currently very narrow in focus.
Grafter: One of the most dangerous monsters. It is a mass of writhing bodies interested only in adding more bodies to itself.