Bughouse Chess is played by two teams of two players each. On each team, one player plays White and the other plays Black. When you capture one of your opponent’s pieces, you hand it to your partner, who can drop it on his own board rather than making an ordinary move. Pawns may not be dropped on the first or eighth rank, but otherwise there are no restrictions on drops; you can even checkmate with a dropped piece. When a pawn reaches the eighth rank, it is promoted normally, but if the promoted piece is captured, it turns into a pawn again. The game ends when one of the kings is checkmated.

Here’s a video of a typical Bughouse game, where the only unusual thing is that one of the players (at the back left) is Levon Aronian, one of the strongest Chess grandmasters in the world, and a participant in the 2007 World Chess Championship.

(text from Nerwisdom.)