difference in penalties

[refinks]

Guys, let me preface this by saying that I'm not a football official, but I've officiated other sports and I'm strongly considering getting some football experience next season, getting my feet wet in some junior leagues or low level middle school games.

Last night I was at a game and something came to my attention: it's always been in the back of my mind when I watch a football game, but I've never really put a lot of thought into it until last night.

Can you guys tell me what is the difference between offsides and encroachment? To me they seem like the same thing, just a 5 yard penalty. But there must be some difference, even a subtle difference. What is it? Thanks in advance.