Invented in 1899 by Henry Michael Temple, this chess variant is played by 2 players but requires a third person to act as an umpire. Essentially, players only know the positions of their own pieces and have to make inferences based on what the umpire tells them (for example, that a move is impossible, or a capture has been made on a specific square).
According to The Encyclopedia of Chess Variants (D.B. Pritchard, 1994) alternative names are Screen Chess, War-Chess and Commando Chess.
David Li's book Kriegspiel: Chess Under Uncertainty (1994) provides a set of rules and some sample games.
Rules and more details are available at The Chess Variant Pages' entry for the game: