Soccer is a simple game. Therefore its rules are not very complicated. Maybe with an exception of the Offside rule, probably the most complex of all the soccer rules.
FIFA, the world soccer governing body, constitutes the official soccer rules. There are 17 fundamental so-called "Laws of the Game". Some of the rules may be modified, especially for games involving kids (under 16 years of age), women, veteran players (over 35) or players with disabilities.
The following modifications are permissible:
Size of the field of play.
Size, weight and material of the ball.
Width between the goalposts and height of the crossbar from the ground.
The duration of the periods of play.
Substitutions.
Further modifications are only allowed with the consent of the International Board.
Of course it applies only to members of FIFA. Any soccer organization, league or club that is not associated with FIFA may have their own rules.
I would like to recommend another great free FIFA's resource: Law of The Game 2005- Questions & Answers
It's a large file, so be patient…it takes a while to open. But it's an awesome reference that will help you fully understand an interpretation of soccer rules.