Diplomacy is a turn-based strategy game set during World War One. The object is to conquer all of Europe.
Each player controls one of the seven powers: Austria, England, France, Germany, Italy, Russia, and Turkey.
Europe is divided into 56 land territories and 19 sea terrirories. Players control armies (shown by large circles) and fleets (shown by triangles), which are assigned to special "supply center" territories (marked with small circles) at the start of the game. Conquering a new supply center allows a player to build a new army or fleet.
In each turn (a spring or fall season), players submit secret orders to move their forces. When all players are ready, or time for the turn runs out, the orders are revealed to see what moves where. After the fall season (that is, every two turns) players can create new armies or fleets if they gained supply centers, or must destroy some if they lost.
Much of the fun and strategy is based on diplomacy: messages sent between players, promising to work together against other players. These messages can be public, so all players see them, or private so only some players see them.