This link has an animated explanation of offside. It only takes a few minutes to run through the entire thing, and you can replay it as many times as you like. The rules have changed slightly since it was produces, but it is still 95% accurate *, and is the most understandable explanation of offside that I've seen.

http://www.dynamic-thought.com/OffsideClickette.html

* The situation described at 4:50 is now considered Offside.

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