Massively Multiplayer Online (Game).

Basically, an ARG is just a lot of puzzle solving and shit. You find yourself interacting with characters that are in-game, even though you're using real methods (email, telephone) to do it. For example, here's an email I got from a character in an ARG I'm playing now:

Help!
More are becoming infected!
We need your help now.
Time is of the essence

274822
296604
472944
003947


Jedilink, if you make that movie, I will love you forever. Platonically. =P There was something on Gamespy about this guy who played an ARG, and his colleagues started messing around with his life to make him all freaked out and stuff. Pretty funny, but I wouldn't want it happening to me.

http://archive.gamespy.com/dailyvictim/index.asp?id=275