Many issues of Flagship carried this description:
What is Play By Mail? The process is simple. Players send their orders for each turn in the game to a central moderator (or GM), who works out the results and sends them back. This simple process allows hundreds of players to enter game worlds of great depth, to contact each other between turns for discussion and negotiation and to play at a time that suits them.
In most games everybody's orders are processed together for each turn, simultaneously, but the results that are sent back are the individual ones for your own position. The other players won't know what you're planning unless you choose to tell them, which offers many possibilities for joint and covert action. There are all sorts of games, with all sorts of settings: roleplaying, wargaming, adventuring, empire-building, sports games and plenty more! Some are simple, some are complex, but they will all take skill and planning to play them well.
If you've never tried Play By Mail, give it a go. It will be unlike any other kind of gaming that you've played.
Also see Play-by-Telephone
Also see Cyberspace Adventures and Cyberspace Heroes