OFFSIDE 

In soccer, the offside rule states that when an offensive player is on the opponent's side of the field and a teammate touches the ball, there must be at least two opposing players who are closer to the goal line than the offensive player is, or that player cannot become involved in the play. For example, a player who has only the opposing goalkeeper between him or her and the goal cannot receive a pass from a teammate. The two defenders closer to the goal line usually include the goalkeeper, but that it not necessarily true. When an official calls a team for being offside, the other team is given possession of the ball for an indirect free kick. 

Timing is Crucial 

This rule was created to prevent offensive players from "cherry picking" near the opponents' goal. Without the rule, offensive players could hover near their opponents' goal even when the play is on the other side of the field, with the hope of a long pass and an easy goal. The offside rule applies at the moment the ball is touched or passed, not when the ball is received. Therefore, if the offensive player who will receive the pass is onside at the time the ball is touched, then runs past the defender before receiving the pass, the receiving offensive player is not offside, and the play is legal. 

230 http://www.wisegeek.org/what-is-offside-in-soccer.htm#didyouknowout