Imagine a game that interacts with players through all different forms of media (TV, radio, outdoor posters, internet, etc.). The whole thing might be a giant puzzle which requires "conscious" players to contact each other or solve clues towards a shared objective. Kinda like the Matrix. Actually, that's a really good example. Neo and company were living in an alternative reality to everyone else, and they had to solve clues, fight agents, do whatever in order to "win."

People who are unaware that the game is going on have no idea. A message board post might tell people in New York to look above a certain window in Times Square at 1pm on Friday, and then it is their job to relay that clue to everyone else, who may receive different clues in different cities. Basically, ARGs are hidden among common, everyday meatspace.

Wiki article here: http://en.wikipedia.org/wiki/Alternate_reality_game

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