If a player is beyond the last defender when the ball is played forward, he is offside if he interferes with play.
Then there's a load of sub-clauses that exist so people can talk about it for hours.
( mike woz ere 7442200 & 7696970 getter, Tue 25 Jan 2011, 18:15, archived)

Such as if he was in his own half when the ball was played.

( Spider Fucking Riviera got stiches for a knife wound for the first time, Tue 25 Jan 2011, 18:19, archived)

Yeah and if the goalkeeper is behind play or the opposition kicks it to the opposite attacker and a load of other ridiculous caveats.
They basically never happen though but when they do I'd take the linesman's decision, male or female, before I listened to Andy Gray's 30 minute analysis of the incident.
( mike woz ere 7442200 & 7696970 getter, Tue 25 Jan 2011, 18:26, archived)

Aye, he's a fucking tit, good enough for him.

( Spider Fucking Riviera got stiches for a knife wound for the first time, Tue 25 Jan 2011, 18:32, archived)