Theory: 21 Kyu to 30 Kyu
Go - The Basic Rules
Actually, and contrarily to what you may think at first glance, Go is a very, very simple game. Or at least the rules are simple.
What is it all about?
Winning or losing is determined by points. Each player tries to amass as many points as possible. These points are counted at the end of the match, and the player with the most points wins - it's that simple.
The game ends once both players agree the game ended and both pass. If a player passes the other one is of course free to continue playing, and the previously passing player may rejoin the game if he chooses to.
How do you play?
A Go-board (or also frequently called "Goban") is made up of crossing lines, usually 9x9, 13x13 or 19x19. These lines cross at intersections.
The two players (black and white) place stones on these intersections one after another, with black beginning. As this gives black an advantage white receives a few points called "komi" in return, usually ranging from 4.5 to 6.5. If an advanced player plays a less advanced one they can use a handicap in order to enable a fair game. This handicap means that black may place several stones (usually on the intersections marked with the little dots, the so called "star points"), before white places his first one.
Each stone has got lines leading away from it ending in other intersections. These neighboring intersections (marked in the image below with squares) are called "liberties".
Two or more stones together can form a group. This group shares its liberties (marked with a triangle). Two or more stones only form a group if those stones are connected vertically or horizontally and not just diagonally.
If all the Liberties are occupied by the opponent's stones, meaning there are no more liberties left for a group, the group is captured. This means the captured stones are taken off the board and kept by the capturer for later counting, for every captured stone is worth one point.
See if you can capture the white stones in the following examples with one move. If there are numerous white groups, only one of those can be captured with the next move. (You are black)
Taking prisoners is not the only way to score points. Taking territory is usually the way more points are being scored than with prisoners. If a players stones entirely surround an area (the border being considered here as belonging to the surrounding player), it makes it his territory. Each intersection within this territory is worth one point. On the picture below you can see black's territory being marked with triangles and white's territory with squares.
White has got 10 + 2 + 8 + 1 + 18 = 39 points and black has got 7 + 16 + 12 + 11 = 46 points. If white had taken two prisoners and black none, the score would be 41 : 46. Now, if you add 4.5 Komi to white's score (on which both players agreed before the game) the score would be 45.5 : 46, meaning that black would win by 0.5 points. Sounds more complicated than it actually is, just play a few games and you'll get the hang of it.
You may have noticed there are some unmarked empty intersections. Those are neutral points and don't belong to anyone.
And that's pretty much it. These are the basic rules. There are, however, a two special rules you have to know about:
Suicide and Ko
Suicide is rarely the solution and in Go it's even forbidden, at least in the usually used Japanese rules.
Black cannot play at 'A', for it would be plain suicide. The placed stone would not have any liberties and would have to be taken of the Goban. However, if Black played at 'B' the marked white stones would not have any liberties anymore and thus would become prisoners. Once the white stones are removed Black's liberties would amount to 4, making 'B' a legitimate move. To sum up: "suicide" is only allowed if it kills your opponent's stones and thus creates liberties for your own stone.
Another special rule concerning "suicide" is Ko. Look at the situation below:
If black played at 'B' he would kill the white stone at 'A'. But now, as it is White's turn, he could kill the bkacl stone at B and we would have the same situation again as in the picture. Black could then again kill White and White could also then kill Black ...
As this could go on forever, it is, similar to chess, forbidden. The exact situation may not occur twice in one game. So after Black killed with B white has to play somewhere else, preferably somewhere where Black must answer. As there is now a different situation on the board White is now allowed to capture the black stone, and Black is forced to play somewhere else.
The End
And that's it. That's all there is to know about the rules. It may now seem pretty abstract, but once you have played a few games the rules come naturally to you. Below you can look at how a real game may actually look if you're interested.
Have fun playing!