Go is a board game played on a 
grid, with pieces being placed at the intersections
of grid lines, rather than in the middle of them. Go boards
have a number of peculiarities, including the fact that the aspect ratio is approximately
0.91 (instead of the usual unity for western games). This is done so that with perspective,
the board appears square. Another is that go pieces (called
stones) are actually slightly larger than the spacing on the board allows, thus requiring
stones to be placed partially underneath one another to fit, producing a state of
mild disorder on a full board.
Go is played by players alternately placing stones on the board, with one player using white stones and the other black stones. There are several sets of rules (Japanese and Chinese) as to how play progresses and scoring is done, but the basic object is to surround as much "territory" as possible. When a group of one player's stones are completely surrounded the other player's (i.e., there are no holes and the stones of one color have stones of the other color adjacent to each possible location horizontally and vertically), the surrounded stones are "captured" by the other player. Go has a number of special terms to refer to specific board configurations, including "atari."
Sensei's Library provides a large collection of go resources, including extensive links to additional material.
The images above show an empty go board and a go board near the end of a game.
There are 
assignments
of empty, black, or white to the intersections of a 
board, giving an upper
bound on the number of legal positions (Beeler et al. 1972, Flammenkamp).
The cumulative numbers of unterminated games with at most 
moves, starting from an empty board and ending with a stone
placement, are 1, 362, 130683, 47046243, ... for 
, 1, 2, ... (OEIS A007565).
The initial empty game is included. Passes are allowed, but two consecutive passes
terminate a game. Suicide moves and stone placements that repeat an earlier board
position are excluded.
