From BRWiki
Play-by-Email or PBEM games have the players submit their actions/moves in the next round or turn via email or similar electronic means to the DM/referee. The referee regularly informs the players what moves were successful, what the consequences of the moves were and any other information relating to the state of the game at the time. This is a modern development of the play-by-mail method sometimes used for games like chess.
There are a number of Birthright campaigns that are run as a PBEM when it comes to domain and court actions.
See Also: Pbem adventure system - Suggestions about how to run "adventures" within a PBEM game.
