It's easy
a player is in an offside position (and may not take an active part in the game) if he finds him or herself between the ball and the goal line, with one member of the opposing team between him or herself and the goal line and at least one member of the opposing team between him or herself and the ball; 'between' in this case being measured purely longitudinally along the pitch.
( Flapjack I spay a little mare for you, Tue 25 Jan 2011, 18:11, archived)

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)

There needs to be two players infront of the attacker to play him onside.
for fuck sake
( Ring Of Fire A petty, drooling belming butthurt retard., Tue 25 Jan 2011, 19:10, archived)