A role-playing game (RPG) is a game of the imagination, where you get to tell stories by taking on the roles of the main characters - characters you create. It's a game that offers a multitude of choices to those characters - the only limit to what you can do is your imagination. The story is like a movie, except all of the action takes place in your mind. There's no script to the movie (other than a rough outline used by the Game Master); you decide what your character says and does. The Game Master (or GM) is the director and special effects designer, deciding what the story is about and taking on the roles of all the other characters. The GM also keeps track of the rules, interprets the outcome of actions, and describes what happens. Together the players and Game Master create a story, and everybody has a good time.
A play-by-e-mail (PBeM) is an RPG game played by electronic mail. In general, a PBeM will go through a various steps: There will be a call for players, character creation, the GM's approval (and rejection) of characters... and, finally, the start of the game.
Most PBeMs, quite naturally, will be played by e-mail. The GM will send a public post to all the players describing the surroundings and current situation. The public post may also contain dialog from non-player characters or summaries of characters' previous actions. If there is some specific information that would only be available to a smaller group or a single player, the GM will send a separate e-mail note to them.
Players, in turn, send a reply via e-mail message to the GM and other players that describes what their character does in the given situation. If the action is not visible to all the character (or is, perhaps, a question to the GM) private e-mail will be sent also.
The cycle of e-mail messages is then repeated and you're off and PBeMing.
Ground Zero is a high-action d20 System RPG in a world devastated by global warfare and overrun by mutants. Ground Zero features weird powers, strange monsters, insane NPCs, and big guns. It's 'high flux', with the promise of great rewards and the constant threat of pointless demise.
Ground Zero has been running as a PBeM since October 2003. It is one part Gamma World, one part Fallout, with a dash of HackMaster thrown in for flavour. The rules are a mish-mash of d20 Modern, Dominic Covey's Darwin's World, and Jonathan Tweet's Omega World - plus stuff from various other games and my own homebrew rules.