The Offsides rule is something that is very difficult to understand at first.  Below are illustrations of how the offsides rule works.

First: You cannot be past the last defender without the ball.  For all images- Red=Defense Yellow=Offense

Second: You can beat the offsides line by playing a pass into the space. There is no offsides until the ball hits another player (defender OR offender)

EXAMPLE:

Third: If the ball does get played into the space behind the defense, the ball is now the offsides line.

But if you move beyond the point of the ball, you will then be considered in an OFFSIDES position.

ADVANCED RULES:

There are two advanced features of the Offsides rule that you may want to know.

First: The rule has to do with what we call "Being involved in the play"

But if the player IS involved with the play, the referee will call offsides.

Second: If the defender plays the ball backwards, the ball becomes the offsides line and negates the offsides call alltogether. 

Similarly, if the defender tries to play it to his GK and the attacker is in an offsides position, the rule is negated.

Note: The rule is subject to interpretation by the Referee's in the game.  The human element causes these rules to not be perfectly enforced.  Please do not harass the referees if they do not make the perfect decision. It is sometimes very difficult to see when the ball is played, and or who touched it last. It is all down to judgement and interpretation by the referee.