A player is in an offside position if they are nearer to their opponents' goal line than both the ball and the second to last opponent.
However, it is not an offence to actually BE in an offside position.
A player in an offside position is only penalised if, (at the moment the ball touches or is played by one of their team) they are involved in active play (i.e. by participating in play, interfering with an opponent or gaining an advantage by being in the aforementioned position.)
See Stuarty, that's MUCH easier to understand!
