Various rules of Go are used around the world. The particular set of rules used generally does not affect how the game is played. We have a summary of the most important rules differences , with links to deeper studies of them.
In Britain, the Japanese rules have traditionally been used. Use of the 1989 version of the rules  could give rise to problems, so in 2006 it was decided to adopt AGA-style rules  as our default rule set for tournaments.
If the organisers of a Go tournament in Britain, recognised by the BGA, do not specify otherwise, then the AGA-style rules as specified in rules of play  apply.
Black gives White 7½ points Komi (under traditional Japanese rules, the Komi was 6 or 6½ points).
Triple kos and other such repeated positions are prevented by a "Superko" rule: It is illegal for a player to play so as to recreate a board position of the game, previously created by a play of the same player. (Under previous BGA rules, repeating the board position led to a jigo.)
Japanese professional games traditionally use the byoyomi system. When a player has only a few minutes left the seconds are counted down, and any move made in less than a minute does not use up any of that player’s time.
In Britain we use the Canadian Overtime system instead. When a player has used all of their main time allocation (typically one hour), they go into overtime. Overtime is made up of an unlimited number of overtime periods.
In each overtime period the player must play a specified number of moves within a specified time period. This number is established by counting out 'overtime stones' from the player's bowl and making these clearly visible to the opponent. The overtime periods might be five minutes in which to play thirty stones.
A move consists of placing a stone on an intersection and removing any consequent captures, or passing, and then pressing the clock. Note that the clock is not stopped for capturing a large number of stones. When passing, an overtime stone should be given to the opponent. A player who fails to play all the overtime stones within the overtime period loses the game on time. The loss is immediate - the opponent does not need to 'claim' a win.
Once all the overtime stones have been played, the clock is stopped. A fresh set of overtime stones is counted out and the clock is reset to the overtime period. Then the clock is restarted.
The above tasks are shared: while one player counts the stones, the other is resetting the clock. Stones should be counted promptly, and whilst the clock is stopped neither player should be analysing the game.
In most tournaments, each overtime period will be identical, with the same number of stones to be played in the same interval. For example, twenty stones in five minutes abbreviated by 20/5 is often used in longer games. In many tournaments the overtime is 30/5 and players can quite happily continue for many overtime periods!
If your tournament is time-critical, i.e. you need to pack up by a given time, you may consider using an accelerated system instead. For example 10/5, then 20/5, then 30/5, then 40/5, and so on, increasing by 10 stones each period.
The overtime period critically affects the tournament class as summarised in our ratings  policy. If you had a main time of 65 minutes and accelerated overtime of 20/5, 40/5, 60/5 increasing by 20 stones each period, the tournament would qualify for class A rating.
A player who arrives late by more than half the main time allowance loses the game by forfeit. The player's opponent gets a free win, and this counts for prizes and pairing, but does not count for rating points.
There are two interpretations to late:
You would use late by start of round if you had a tight time schedule. If you do use late by start of clock then be aware that in the extreme case Black could arrive 29 minutes late for a 60 minute game and then White could arrive a further 29 minutes late. The game would effectively start when other games had been going for an hour!
Any dispute by a player in either their own game or in another game must be communicated to the referee as soon as possible and in any case before the results of the game are published.
On-lookers are strongly discouraged from making any comments of any kind about a game in progress. If there is a suspected rule violation, then this should be brought to the attention of the referee, and should never be discussed with the players.