Play by mail games are games, of any type, played through postal mail or e-mail. One example, chess, has been played by mail for centuries (when played in this way, it is known as correspondence chess). Another example, Diplomacy has been played by mail since the 1960s, starting with a printed newsletter (a fanzine) written by John Boardman. More complex wargames, moderated by computer programs, were pioneered by Rick Loomis and his company, Flying Buffalo Games.
Today, many games are played by e-mail as well as postal mail. In addition, some computer games can be played in a play by mail mode: you make your "move," mail a file to your opponent, who makes his "move," and mails something back.
Play by mail games differ from popular online multiplayer games in that, for most computerized multiplayer games, the players have to be online at the same time. With a play by mail game, the players can play whenever they choose.
Play by mail games are often referred to as PBM games, as opposed to face to face (FTF) games, which are played in person.
E-Mail Games is a site that offers free wargaming by e-mail.