Beer pong, also known as Beirut, is the staple game of US college parties. It can now be considered a sport - it has its own world series! It is a drinking game in which players throw a ping pong ball across a table to try and land the ball in a cup of beer at the other end.
The official site for beer pong start their list of rules with "Rules are made to be broken". That is a sport which does not take itself too seriously. There are many variations of the game, and they are all OK. For the purpose of official competition (such as the World Series of Beer Pong), some standard rules had to be set down. For any game, the rules need to be agreed upon before starting play. Often there are house rules that need to be followed.
Generally, the game is often played with teams of 2-4, with six or ten plastic cups arranged in a triangle at each end. Players throw the ping pong ball, trying to land it in an opponents cup. Once the ball has been sunk in a cup, that cup is then eliminated from the game and the opponent consumes the cup's contents.
The rules used for the World Series includes 2 players per team, 2 balls per game, 2 shots per side (except first toss) and 10 cups per side. Also, players do not have to play with beer - they can choose to play with water at their discretion.
The World Series of Beer Pong VIII was held in Vegas in January 2013, with 160 teams attending. Some take it pretty serious, as you would expect for $50,000 first prize. Some teams opted to fill some of their cups with water and competition organizers only filled four of the ten cups with liquid. The winning team was team called “Drinkin Smokin Straight West Coastin” from California.