American-style eight-ball rules are played around the world by professionals, and in many amateur leagues. The rules for eight-ball may be the most contested of any billiard game. There are several competing sets of "official" rules. The non-profit World Pool-Billiard Association (WPA), with national affiliates such as the Billiard Congress of America (BCA), promulgates the World Standardised Rules for amateur and professional play. The for-profit International Pool Tour has also established an international set of rules for professional and semi-professional play, used in major tournaments broadcast on television. Meanwhile, many amateur leagues, such as the American Poolplayers Association (APA) / Canadian Poolplayers Association (CPA), and the Valley National Eight-ball Association (VNEA) / VNEA Europe, use their own rule sets as their standards (most of them at least loosely based on the WPA/BCA version), while millions of individuals play informally using colloquial rules which vary not only from area to area but even from venue to venue.

A summary of the international rules follows (see the WPA/BCA or IPT published rules, which conflict on minor points, for more details):

Equipment: There are seven solid-colored balls numbered 1 through 7, seven striped balls numbered 9 through 15, an 8 ball, and a cue ball.

The balls are usually colored as follows:

• 1 and 9 - yellow
• 2 and 10 - blue
• 3 and 11 - red
• 4 and 12 - purple
• 5 and 13 - orange
• 6 and 14 - green
• 7 and 15 - brown
• 8 - black
• cue - white.

The table's playing surface is approximately 9 ft. by 4.5 ft. (regulation size), though some leagues/tournaments may allow other sizes.

Setup: To start the game, the object balls are placed in a triangular rack. The base of the rack is parallel to the end rail (the short end of the pool table) and positioned so the apex ball of the rack is located on the foot spot. The balls in the rack are ideally placed so that they are all in contact with one another. This is accomplished by pressing the balls together from the back of the rack toward the apex ball. The placement of the balls, for a legal rack according to World Standardised Rules is that the 8 ball is placed in the center, while the two lower corners must be a stripe and a solid. The cue ball is placed anywhere the breaker desires inside the "kitchen".

Break: One person is chosen (by a predetermined method, e.g., coin flip, win or loss of previous game, lag) to shoot first and break the object ball rack apart. If the shooter who breaks fails to make a legal break (usually defined as at least four balls hitting cushions or an object ball being pocketed), then the opponent can demand a re-rack and become the breaker, or elect to play from the current position of the balls.

If the breaker pockets a ball, it is still that player's turn and the table is considered "open" (meaning the breaker can still make any object ball to determine if he/she will only shoot solids or stripes throughout the game). If the breaker fails to make another ball after the break, the table is still considered "open" until someone legally pockets a ball.

According to World Standardized Rules, if the 8 ball is pocketed on the break, breaker may ask for a re-rack or have the 8 ball spotted and continue shooting. If the breaker scratches while pocketing the 8 ball on the break, the incoming player has the option of a re-rack or having the 8 ball spotted and begin shooting with ball-in-hand behind the head string.

Turns: A player (or team) will continue to shoot until committing a foul ( fault), or failing to pocket a legal object ball. Then it is the turn of the opposing player(s). Play alternates like this for the remainder of the game. After a foul, the incoming player has ball-in-hand anywhere on the table.

Pocketing the 8 ball: Once all the player's object balls are pocketed, he/she may attempt to sink the 8 ball. To win, the player must first call which pocket they plan to sink the 8 ball into. If the 8 ball is shot into the wrong pocket or a foul (see below) occurs, the player loses. Otherwise, the player's turn is over.

Winning

• The player has legally pocketed the 8 ball, after all his/her object balls have been pocketed
• The opposing player illegally pockets the 8 ball (e.g. before clearing all of his/her object balls, in the same shot as the last such object ball, or into a pocket other than the one that was called)
• The opposing player scratches the cue ball into a pocket, or knocks it off of the table, when the eight ball is pocketed. A scratch or foul is not loss of game if the 8 ball is not pocketed or jumped from the table
• The opposing player commits any foul on the shot that pocketed the 8 ball (in non-tournament situations, non-cue-ball fouls may be excused from this requirement)
• The opposing player knocks the 8 ball off of the table.

Fouls

• The shooter fails to strike one of his/her own object balls (or the 8 ball, if all of said object balls are already pocketed) with the cue ball, before other balls (if any) are contacted by the cue ball. This includes "split" shots, where the cue ball strikes one of the shooter's and one of the opponent's object ball simultaneously.
• No ball comes into contact with a cushion or is pocketed, after legal cue ball contact with the (first) object ball (or 8 ball).
• The cue ball is pocketed ("scratched")
• The shooter does not have at least one foot on the floor (this requirement may be waived if the shooter is disabled in a relevant way, or the venue has not provided a mechanical bridge)
• The cue ball is shot before all balls have come to a complete stop from the previous shot
• The cue ball is struck more than once during a shot
• The cue ball is jumped entirely or partially over an obstructing ball with an illegal jump shot that scoops under the cue ball
• The cue ball is clearly pushed, with the cue tip remaining in contact with it more than momentarily
• The shooter touches the cue ball with something other than the tip of the cue
• The shooter touches any other ball (with body, clothing or equipment), other than as necessary to move the cue ball when the player has ball-in-hand
• The shooter knocks a ball off of the table
• The shooter has shot out-of-turn.
• On the break shot, no balls are pocketed and fewer than four balls reach the cushions (in which case the incoming player can demand a re-rack and take the break or force the original breaker to re-break, or may take ball-in-hand and shoot the balls as they lie)

This information was modified from: http://en.wikipedia.org/wiki/Eight_ball