Offside is one of the most controversial laws in soccer, primarily because of misconceptions about exactly what it means. Here is a comprehensive offside primer for parents used by a local youth league. Another excellent resource is a flash animation put together by FIFA explaining the offside law. Go to the FIFA Laws of the Game web page and click 'Interactive Guide to Law 11'.
Offside is often not enforced at younger age levels. Also the proper term is offside as in 'the player was offside'. Saying 'offsides' around an experienced youth soccer parent or coach will probably make the twitch. :)