Diretamente da Wikipedia:
Beer Pong (also known as Beirut) is a drinking game in which players throw a ping-pong ball across a table with the intent of landing the ball in a cup of beer on the other end. The game typically consists of two two-player teams and multiple cups set up on each side set up in triangle formation. There are no official rules, so rules may vary widely, though usually there are six or ten plastic cups arranged in a triangle on each side.
The goal of the game is to eliminate the other teams' cups before one's own cups are eliminated. When a ball lands in a cup, which are generally 1/4 to 1/3 full, that cup is eliminated and the defending team must consume all of the beer inside that cup.
The losing team must consume all the beer remaining in the winning team's cups. The order of play varies – both players on one team shoot followed by both players on the other team, or players on opposite teams can alternate back and forth.