Internet Diplomacy refers to any of a number of online implementations of Diplomacy, a board game in which seven players, each controlling one of the major European powers of the early 20th century, fight for control over Europe.
Instead of communicating face-to-face, as in the board games, or by mail, as in the play-by-mail games, communication and order submission is done over the internet. Typically orders are processed by software, but in the earlier systems a person called a GameMaster would process the orders manually.
Internet Diplomacy is popular for many reasons, including avoiding the hassles of playing the game face-to-face, and the lack of availability of local players. Also, a face-to-face game needs up to seven players and can last several hours, which makes it difficult to find enough players with enough free time for a game.
Internet Diplomacy brings together players from anywhere in the world, with deadlines ranging between a few minutes to a few days, and allows players to play multiple games at the same time. The role of GameMaster, depending on the specific implementation, can be minimized to setup and resolution of rules questions/disputes, or even eliminated entirely.
This makes Internet Diplomacy the preferred choice for many, but it is not without its flaws. Plain text messages aren't as good as face-to-face conversation for back-and-forth discussion or watching for signs of deception. Also playing with relatively anonymous remote players may be less emotionally rewarding than playing with close friends.
Play-by-email internet Diplomacy followed from Play-by-mail games dating to 1970s fan zines. Similar to standard mail games, players originally sent press directly to one another through email, and sent their orders via email to a pre-designated human GameMaster.
As email became more prevalent software Judges were developed, which would route emails addressed from one player to another, and route submitted orders and results to/from human GameMaster(s). As well as routing messages they provided more accountability and security, a place where all the info about the game could be stored and retrieved, and provided a way to keep scores of players from multiple games.
These software Judges were eventually extended to include adjudicators, which are pieces of software capable of resolving orders and producing results, thus replacing the human GameMaster with software. Those adjudicators often had many bugs and only after a few years they became reliable. The Diplomacy Adjudicator Test Cases were written to overcome this problem. With the use of these test cases new programs can be of high quality on first release. The DATC contains also recommendations for ambiguities in the rules.