FIFA have a very good animated explanation detailing what offside is on their website. It covers what constitutes active play in an offside position and what the exceptions to offside are.
It’s prefaced with this comment, “Law 11 (offside) is one of the shortest of the 17 laws, but it is perceived to be one of the most complicated and controversial. In reality, this is not the case; Law 11 is very easy to understand.”
…You then get treated to 37, mainly animated slides explaining the oh so simple offside rule. It’s actually very good. I just have no idea why FIFA need to try to sell the offside rule as being simple. It’s not as witnessed by the mistakes in interpretation by their own referees at the World Cup. I don’t mean when they get it wrong on marginal calls where it’s hard to tell if the attacker is level or in front of the second last defender. I mean situations like the Italy v New Zealand game where Cannavaro plays the ball, unintentionally, to a New Zealand player in an offisde position who benefits from that position (by scoring) and its not called. That scenario is covered in FIFA’s explanation of offside but clearly the ref in that game was not aware of that aspect of the offense or else the goal would not have stood…and Italy would likely have advanced to the knockout phase.