Snooker is a cue sport that is played on a large green baize-covered table with pockets in each of the four corners and in the middle of each of the long side cushions. A regular (full-size) table is 12 × 6 ft (3.7 × 1.8 m). It is played using a cue and snooker balls: one white cue ball, 15 red balls worth one point each, and six balls of different colours: yellow (2 points), green (3), brown (4), blue (5), pink (6) and black (7). A player (or team) wins a frame (individual game) of snooker by scoring more points than the opponent(s), using the cue ball to pot the red and coloured balls. A player wins a match when a certain number of frames have been won.

Snooker, generally regarded as having been invented in India by British Army officers, is popular in many of the English-speaking and Commonwealth countries, with top professional players attaining multi-million pound career earnings from the game. The sport is particularly and increasingly popular in China.

It is commonly accepted that snooker originated in the later half of the 19th century. Billiards had been a popular activity amongst British Army officers stationed in India, and variations on the more traditional billiard games were devised. One variation, devised in the officers' mess in Jabalpur during 1874 or 1875, was to add coloured balls in addition to the reds and black which were used for pyramid pool and life pool. The word snooker also has military origins, being a slang term for first-year cadets or inexperienced personnel. One version of events states that Colonel Sir Neville Chamberlain of the Devonshire regiment was playing this new game when his opponent failed to pot a ball and Chamberlain called him a snooker. It thus became attached to the billiards game now bearing its name as inexperienced players were labelled as snookers.

The game of snooker grew in the latter half of the 19th century and the early 20th century, and by 1927 the first World Snooker Championship had been organised by Joe Davis who, as a professional English billiards and snooker player, moved the game from a pastime activity into a more professional sphere. Joe Davis won every world championship until 1946 when he retired. The game went into a decline through the 1950s and 1960s with little interest generated outside of those who played. In 1959, Davis introduced a variation of the game, known as snooker plus, to try to improve the game's popularity by adding two extra colours. However, it never caught on.

A major advance occurred in 1969, when David Attenborough, who was then a top official of the BBC, commissioned the snooker tournament Pot Black to demonstrate the potential of colour television, with the green table and multi-coloured balls being ideal for showing off the advantages of colour broadcasting. The TV series became a ratings success and was for a time the second most popular show on BBC Two. Interest in the game increased and the 1978 World Championship was the first to be fully televised. The game quickly became a mainstream sport in the UK, Ireland and much of the Commonwealth and has enjoyed much success in the last 30 years, with most of the ranking tournaments being televised. In 1985 a total of 18.5 million viewers watched the concluding frame of the world championship final between Dennis Taylor and Steve Davis. In recent years the loss of tobacco sponsorship has led to a decrease in the number of professional tournaments, although some new sponsors have been sourced; and the popularity of the game in the Far East and China, with emerging talents such as Liang Wenbo and more established players such as Ding Junhui and Marco Fu, bodes well for the future of the sport in that part of the world.

The object of the game is to score more points than the opponent by potting object balls in a predefined order. At the start of a frame, the balls are positioned as shown and the players take it in turns to hit a shot in a single strike from the tip of the cue, their aim being to pot one of the red balls and score a point. If they do pot at least one red, then it remains in the pocket and they are allowed another shot – this time the aim being to pot one of the colours. If successful, then they gain the value of the colour potted. It is returned to its correct position on the table and they must try to pot another red. This process continues until they fail to pot the desired ball, at which point their opponent comes back to the table to play the next shot. The game continues in this manner until all the reds are potted and only the 6 colours are left on the table; at that point the aim is then to pot the colours in the order yellow 2, green 3, brown 4, blue 5, pink 6, black 7. When a colour is potted in this phase of a frame, it remains off the table. When the final ball is potted, the player with the most points wins.

Points may also be scored in a game when a player's opponent fouls. A foul can occur for numerous reasons, such as hitting a colour first when the player was attempting to hit a red, potting the cue ball, or failing to escape from "a snooker" (a situation where the previous player finished their turn leaving the cue ball in a position where the object ball cannot be hit directly). Points gained from a foul vary from a minimum of 4 to a maximum of 7 if the black ball is involved.

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 predefined number of frames and the player who wins the most frames wins the match overall. 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.

Professional and competitive amateur matches are officiated by a referee who is the sole judge of fair play. The referee also respots the colours on to the table 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.

Other terminology used in snooker includes a player's break, which refers to the total number of consecutive points a player has amassed (excluding fouls) when at one visit to the table. 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 to pot all reds with blacks then all colours, which would yield 147 points; this is often known as a "147" or a "maximum".

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' x 16' or approximately 5 m x 7 m), which is the minimum required for comfortable cueing room on all sides.[20] 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' x 5', 9' x 4.5', 8' x 4', 6' x 3' (the smallest for realistic play) and 4' x 2'. Smaller tables can come in a variety of styles, such as fold away or dining-table convertible.