With many of our players (and parents) spending their winters playing hockey, the offside rule is often misinterpreted. There is no red line in soccer, and often being offside is not a black and white decision but dependent on a number of factors.
The basic rule is that a player is in an offside position if there are not 2 defenders (including the goalie) or the ball between him and the goal line. Being in an offside position does not necessarily make the player offside. The offside is determined at the time of the pass to the player. The player can move to an offside position while the ball is in motion towards him. The player also needs to be part of the play, and there are a few exceptions for goal kicks, throw ins and corners.
For more information and some excellent examples, go to the BBC Sports Academy.