A Go board (goban in Japanese) is a grid with 19 horizontal and 19 vertical lines, forming 361 intersections. For beginning players or short games, smaller boards of 13x13 or 9x9 intersections are sometimes used, without otherwise changing the rules.
Playing pieces consist of two sets of stones (go-ishi), one black set and one white set. The number of stones is indefinite (the rules assume an endless supply) but 181 black stones and 180 white stones are sufficient to cover the 361 intersections of the board, so these amounts are usually found in a full set.
Since the number of stones is large, they are stored in bowls (go-ke), one for each player; this usually has a lid which, upturned before play, is used to hold captured stones.
Go is a game for two players. One player uses the black stones, the other white. The board starts empty. Black moves first (this gives a slight advantage, so the weaker player traditionally plays Black; a handicap can be used to give Black several starting moves, see below). (In ancient China, it was White who moved first.)
The players alternate making moves. Making a move consists of putting a single stone on one of the intersections (the intersections at the edges and corners of the grid are part of the board and plays there are also valid). Once played, a stone does not move, and remains at the same point unless it is captured. A player is allowed to pass instead of making a move. A player may also resign on his move, conceding victory to the opponent.
Stones of the same color that are directly adjacent (along the lines of the board) are said to be connected into a string. Stones on the board have a certain number of adjacent empty intersections, called liberties. When a stone, or string of stones, has no remaining liberties, it is captured. The entire string is taken off the board, and added to the opponent's prisoner pile.
To prevent endlessly repeating positions, the rule of ko (a Japanese word for "eternity") prevents any play that would repeat the previous board position.
This occurs most often when a stone has just been captured, and the stone which made the capture is left with only one remaining liberty. If recapturing that stone would recreate the same board position from the previous move, the position is called ko, and the recapturing move is illegal. The rule of ko states that a player may not recapture on their next move and must play elsewhere. After a new move, the board position will be different, and the stone may then be recaptured.
If a stone is played such that it has no remaining liberties (or is part of a string with no remaining liberties) and does not gain liberties by immediately capturing an opponent's string, this is termed suicide since the stone, and any stones it was connected to, would be thus reduced to zero liberties and captured. Though it is usually a mere tactical blunder to do so, many rule sets prohibit a suicide play, making it an invalid move.
Note that suicide of a single stone (playing on an empty point completely surrounded by the opponent) would cause the board position to repeat, and would thus be invalid under the Ko rule even when using rules that permit suicide.
When both players have passed, the game has ended. Dead stones (those that remain on the board but cannot avoid capture) are now removed as if they were captured. Most rule sets allow disputes over the status of strings and loosely-connected groups of strings to be resolved simply by continuing play until both players are agreed. The Japanese rules, instead, have a long list of exceptions and precedents that are referred to in tournament play. Most players remain unaware of these complications in the Japanese rules for the vast majority of their games.
After dead stone removal, counting begins to determine which player is the winner with the greater share of the board. There are two methods of scoring. In the Japanese, or territory scoring method, each player scores the number of empty intersections he has enclosed, and subtracts the number of captures taken from him (this is done easily by placing the captures taken from a player into their empty intersections to reduce the score). In the Chinese, or area scoring method, captures are not scored, but a player scores for every intersection that he controls -- that is, all points where he has placed a stone or that are completely surrounded by his stones.
Whichever scoring method is used, the player with most points wins. In normal circumstances, the Chinese and Japanese scoring methods give the same winner.
To allow players of different skills to compete fairly, handicaps and komi are used. These are considered a part of the game, and unlike in many games they do not distort the nature of the game. Players at all levels of strength employ handicaps to make the game more balanced.
Handicaps are given by allowing the weaker player to take Black, and declaring White's first few moves as mandatory "pass" moves. In practice, this means that Black's first move is to place a set number of stones (usually the number is equal to the difference in the players' ranks) on the board before allowing White to play. Traditionally, the "star points" -- strategically-important intersections marked with small dots -- are used to place these handicap stones. On the 19x19 board, there are nine star points: at the four 4-4 points in the corners, at the four 4-10 points along the sides, and one at the 10-10 point (the centre of the board, or tengen in Japanese). Other board sizes do not necessarily have marked star points.
When Black is only one rank weaker (also known as one stone weaker, due to the close relationship between ranks and the handicap system) he is given the advantage of playing Black, but without any mandatory White passes. For rank differences from two through nine stones, the appropriate number of handicap stones are used. Beyond nine stones, the difference in strength between the players is usually considered great enough that the game is more a lesson, with White teaching Black, so nine stones is the nominal upper limit on handicap stones regardless of the difference in rank (though higher numbers of stones, like thirteen or seventeen, can be given if the teacher wants more of a challenge).
In an "even", or non-handicap game, Black's initial advantage of moving first can be offset by komi (compensation points): a fixed number of points, agreed before the game, added to White's score at the end of the game. The correct value of komi (to properly compensate for Black's advantage) is controversial, but common values are 5.5, 6.5 or 7.5 -- the fractional value avoids a tied game. In a handicap game, komi is usually set to 0.5 (i.e. White wins if the game is tied). A handicap game with a handicap of 1 starts like an even game, but White receives only 0.5 komi (i.e. a White player who is stronger by one rank is handicapped only by Black's first-move advantage).
There are many official rule sets for playing Go. These vary in significant ways, such as the method used to count the final score, and in very small ways, such as whether the two kinds of "bent four in the corner" positions result in removal of the dead stones automatically at the end of the game or whether the position must be played out, and whether the players must start the game with a fixed number of stones or with an unbounded number.
Rule sets include AGA (American Go Association), Chinese, Japanese, Korean, French, New Zealand, and various modifications such as those for simple games, IGS (Internet Game Server), Ing Chang-Ki's rules, etc.
Further detailed information may be found at the following external links. Note that no one link has a complete list of all commonly used rule sets, and most of these links do not have complete information any one rule set. However, full information can be found by traversing links located at these Web sites.