1 Answer

Basically, a player is offside if:

He is in the opponents half of the pitch
He is in front of the ball
The ball is last touched by another player on his team
There are less than two opposing players (can include the goalkeeper) who are level with him or in advance of him on the pitch, and
He receives a pass or interferes with play.

The rule is complicated, but essential for the game. There have been different forms of the offside rule for the last two hundered years. Without it, players used to split into two groups; attackers and defenders and hang around the goalmouths. When a defender got the ball he kicked it upfield to the opposing goalmouth - play was merely a series of goalmouth scrambles at either end.