One of the most difficult rules to learn in all team sport, Law 11 offside, is misunderstood by a large number of people. From beginning players and casual fans to experienced players and coaches, whether a player is offside is a bone of contention at least once in every match. Aware of this problem, the State Referee Committee spends considerable time in the initial certification course for new referees and in the annual re-certification of referees covering offside play and its components.
Offside is simple enough in its basic concept, an attacking player not in possession of the ball in the attacking half of the field cannot be closer to goal than the second to last defender. This law is in place to prevent unfair advantage in play and has long been a part of the game. The oldest known written form of the offside rule is in the library of the Shrewsbury School in England and dates to 1856. The 1856 rule isn't quite the same as we have today, that coming in around 1925. If Law 11 has been largely the same for almost 100 years and is simple enough in concept why does it cause so much trouble? The three main reasons are that the offside line moves, confusion as to what constitutes being in an offside position and a player being in an offside position does not necessarily constitute offside play.
The following video is a very good explanation of these key points. Like the nature of Law 11 itself, the video ends by making a statement that will actually lead to some confusion. After watching it, read on below for a final clarification.
The video ends by stating that a player is still offside if the ball deflects off a defender. This is both true and false depending on what you call a deflection. If an attacking player strikes a ball and it deflects of the defender to a player in an offside position without the defender intentionally trying to play the ball then the play is offside.
If in the same situation the defender does play the ball but it "deflects" off the defender and takes an unintended direction to the player that had been in the offside position, the play is considered onside. The play here resets because a key component to being in an offside position is that your team has possession of the ball. In this example the defending team gained possession, although very briefly and unsuccessfully, when the defender played the ball.