The Offside Rule and Offside Trap in Soccer
It is not an offence in itself to be in an offside position.
A player is in an offside position if:
he is nearer to his opponents' goal line than both the ball and the second last opponent
A player is not in an offside position if:
he is in his own half of the field of play
he is level with the second last opponent
he is level with the last two opponents
Commiting an Offside Offence
A player in an offside position is only penalised if, at the moment the ball touches or is played by one of his team, he is, in the opinion of the referee, involved in active play by:
interfering with play
interfering with an opponent
gaining an advantage by being in that position
No Offence
There is no offside offence if a player receives the ball directly from:
a goal kick
a throw-in
a corner kick
Infringements/Sanctions
For any offside offence, the referee awards an indirect free kick to the opposing team to be taken from the place where the infringement occurred.
This is offside because the red number 10 is in front of all of the defenders, leaving only the goalkeeper back which isn't enough players to play him onside. This position may have been forced by the defenders moving forward in what is called the offside trap.
Here we can see that the blue number 3 defender has fail to move up the field with rest of his defence and played the red number 9 onside. This is a classic example of where the offside trap fails
The offside rule exists to stop goal hanging, where a player stands next to the opposing teams goal keeper in the hope that someone can get the ball to him (probably using a long ball), so he can get it past the goal keeper. Which would make for a very boring game.
The red number 9 isn't beyond the blue 4 defender but is offside because the goal keeper isn't back. This is one of those rare occasions where goal keeper is out of the goal (e.g. last minute of F.A. Cup final on a corner kick) and can't get back in time but if the attacking team play the ball as they normally would, then it would be offside because the offside rule requires two defenders to be in front the attacker and the goal keeper usually counts as a defender.
Why are linesmen (assistant referees) always calling offside when it isn't? Well believe it or not its not really their fault (unless the decision goes against you or your team), it's all to do with angles and line of sight. In the picture the purple line represents the linesman's line of sight which as you can see is at a slight angle, this line should be parallel with the goal line. So even though the red number 10 is being played onside by the blue defender it will be called offside. This is unfortunate but does happen from time to time.