ONCE AND FOR ALL learn what offside means in soccer.

It's a controversial rule because the referees get freedom of interpretation:

In soccer, the offside rule states that when an offensive player is on the opponent's side of the field and a teammate touches the ball, there must be at least two opposing players who are closer to the goal line than the offensive player is, or that player cannot become involved in the play. For example, a player who has only the opposing goalkeeper between him or her and the goal cannot receive a pass from a teammate. The two defenders closer to the goal line usually include the goalkeeper, but that it not necessarily true. When an official calls a team for being offside, the other team is given possession of the ball for an indirect free kick.

Timing is Crucial

This rule was created to prevent offensive players from "cherry picking" near the opponents' goal. Without the rule, offensive players could hover near their opponents' goal even when the play is on the other side of the field, with the hope of a long pass and an easy goal. The offside rule applies at the moment the ball is touched or passed, not when the ball is received. Therefore, if the offensive player who will receive the pass is onside at the time the ball is touched, then runs past the defender before receiving the pass, the receiving offensive player is not offside, and the play is legal.

Setting a Trap

A common strategy for the defensive team is to play what is called an offside trap, in which the defenders try to draw the opponent offside by running forward right before a potential pass, thereby causing the furthest forward offensive player to be closer to the goal line than the second-to-last defender. The offside trap can backfire, however, if the defenders do not get ahead of the receiving offensive player before the pass is made or if the officials do not call offside. If the receiving offensive player is even — or "level" — with the defenders at the moment of the pass, the player is onside, which can result in the receiving player being wide open if the defenders continue to move away from the goal line.

Exceptions

There are some exceptions to the rule of offside. It cannot be called if the offensive players are on their own side of the field. Nor does it apply on a throw-in, goal kick or corner kick.

I thought it was funny when Portugal was down by one, they had 5 guys offsides at one time on one play!

They were hopped up on adrenaline, eager for that score.

There's no more penalty for 5 guys offsides than 1 guy.

It's really easy to understand:

The key to identifying offsides is not just one or more attacking players being in an offsides position, but also being involved in the play in a way that creates an advantage for the attacking team (e.g. receiving the ball directly), or a disadvantage for the defending team (e.g. by obstructing a defender from getting back into the play)...  

The referee (running the center of the field) depends upon his two linesmen (assistant referees running up and down each half of their sidelines) to determine if an offsides infraction occurs on an attack.

It's much easier seeing offsides in action then it is explaining it on paper... especially with today's televised World Cup games showing replays with graphic overlays onscreen.

Right now is the best time to learn while enjoying the most beautiful game!  This World Cup has been the most exciting and competitive in several decades!