The game

Snooker table with balls placed in their starting positions. At the start of the game, the cue ball (white) may be placed anywhere in the semicircle, known as the "D".

The object of the game is to score more points than one's opponent by potting object balls in the correct order. At the start of a frame, the balls are positioned as shown, and the players then take turns to hit shots by striking the cue ball with the tip of the cue, their aim being to pot one of the red balls into a pocket and thereby score a point, or, if this is not possible, to at least hit a red ball so as to avoid making a foul shot. After the striker pots a red ball, he or she must then pot one of the six "colours" (in snooker, the term colour is understood to exclude the red balls). If the player successfully pots a colour, the value of that ball is added to the player's score, and the ball is returned to its correct position on the table. After that, the player must pot another red ball, then another colour, and so on. This process continues until the striker fails to pot the desired ball, at which point the opponent comes back to the table to play the next shot.

Game in progress on a half-size table. A red ball about to be potted.

The game continues in this manner until all the reds are potted and only the six colours are left on the table. At this point the colours must be potted in the order from least to most valuable ball – that is, yellow first (2 points), then green (3 points), brown (4 points), blue (5 points), pink (6 points) and finally black (7 points), the balls not being returned to play. When the final ball is potted, the player with more points wins. If the scores are equal when all the balls have been potted, the black is placed back on its spot as a tiebreaker. A player may also concede a frame while on strike if he or she thinks there are not enough points available on the table to beat the opponent's score. In professional snooker this is a common occurrence.
Points may also be scored in a game when a player's opponent fouls. A foul can occur for various reasons, most commonly for failing to hit the correct ball (e.g. hitting a colour first when the player was attempting to hit a red), or for sending the cue ball into a pocket. The former may occur when the player fails to escape from "a snooker" – a situation in which the previous player leaves the cue ball positioned such that no legal ball can be struck directly without obstruction by an illegal ball. Points gained from a foul vary from a minimum of 4 to a maximum of 7 if the black ball is involved.
The total number of consecutive points (excluding fouls) that a player amasses during one visit to the table is known as a "break". A player attaining a break of 15, for example, could have reached it by potting a red then a black, then a red then a pink, before failing to pot the next red. The traditional maximum break in snooker is achieved by potting all reds with blacks then all colours, yielding 147 points; this is often known as a "147" or a "maximum". The highest possible break is a 155 break, also known as a super maximum. This is achieved via the opponent leaving a free ball, with the black being potted as the additional colour, and then potting 15 reds and blacks with the colours. Jamie Cope has the distinction of being the first player in snooker history to post a verified 155 break, achieved in a practice frame in 2005.
One game, from the balls in their starting position until the last ball is potted, is called a frame. A match generally consists of a predetermined number of frames and the player who wins the most frames wins the match. Most professional matches require a player to win five frames, and are called "Best of Nine" as that is the maximum possible number of frames. Tournament finals are usually best of 17 or best of 19, while the World Championship uses longer matches – ranging from best of 19 in the qualifiers and the first round proper, up to 35 frames in length (first to 18), and is played over two days, extended if necessary until a winner is determined.
Professional and competitive amateur matches are officiated by a referee who is the sole judge of fair play. The referee also replaces the colours on the table when necessary and calls out how many points the player has scored during a break. Professional players usually play the game in a sporting manner, declaring fouls the referee has missed, acknowledging good shots from their opponent, or holding up a hand to apologise for fortunate shots, also known as "flukes".

An extended spider, which can be used to bridge over balls obstructing a shot that is too far away to be bridged by hand

Accessories used for snooker include chalk for the tip of the cue, rests of various sorts (needed often, due to the length of a full-size table), a triangle to rack the reds, and a scoreboard. One drawback of snooker on a full-size table is the size of the room (22 by 16 feet (6.7 m × 4.9 m)), which is the minimum required for comfortable cueing room on all sides. This limits the number of locations in which the game can easily be played. While pool tables are common to many pubs, snooker tends to be played either in private surroundings or in public snooker halls. The game can also be played on smaller tables using fewer red balls. The variants in table size are: 10 ft × 5 ft, 9 ft × 4.5 ft, 8 ft × 4 ft, 6 ft × 3 ft (the smallest for realistic play) and 4 ft × 2 ft. Smaller tables can come in a variety of styles, such as fold away or dining-table convertible.