Bughouse is a highly addictive variant of chess. The participants are 4 players divided into 2 teams. When one player captures a piece he passes it to his partner; the piece is now said to be in the partner's hand. Instead of a normal move you may drop the piece on the board by placing it on an unoccupied square.
Bughouse is always played with a chess clock for the following reason: if you are about to be checkmated, you shouldn't move as your partner can still win (this is known as sitting). However since the games are timed you can only use this stategy if you have more time left on your clock than your partner's opponent. Most over the board games are played with 5 minutes. Internet games are played with 3 or less minutes (and a 0 increment).
Bughouse requires a much different stategy than chess. Here are some of the more important points:
Time is very important. If you're oppenents spend less time on their moves than they can sit whenevery they want. If your oppenent wants a certain piece he can simply sit and wait for his partner to capture it, as long as he is still up on time.
Mating attacks are much stronger. Since you have pieces in your hand it is much easier to start a mating attack. This makes attacking and defending both much more important.
Communication plays a big role. You should always know what is going on your partner's board so you know what pieces you can give away. You should always keep your partner informed of your position for the same reason.
The word Bughouse was originally used as a term for an insane asylum.