Good definition of ARG:
“Alternate Reality Games take the substance of everyday life and weave it into narratives that layer additional meaning, depth, and interaction upon the real world. The contents of these narratives constantly intersect with actuality, but play fast and loose with fact, sometimes departing entirely from the actual or grossly warping it – yet remain inescapably interwoven. Twenty-four hours a day, seven days a week, everyone in the country can access these narratives through every available medium – at home, in the office, on the phones; in words, in images, in sound. Modern society contains many managed narratives relating to everything from celebrity marriages to brands to political parties, which are constantly disseminated through all media for our perusal, but ARGs turn these into interactive games. Generally, the enabling condition to is technology, with the internet and modern cheap communication making such interactivity affordable for the game developers. It’s the kind of thing that societies have been doing for thousands of years, but more so. Much more so.”

Martin, A et al ~ Introduction to ARG (29.10.2009)