"(...) a fictional genre that unites real life, treasure hunting, live-action, games and online communities. These games are an intense series of puzzles involving websites, clues in newspapers, phone calls from other characters and much more."
Dave Szulborski, author of "This is not a game, a guide to ARGs"
Companies are already using mechanics of this tool in their communication strategies. If you want to know more about ARGs access this site http://argn.com/ and see the excellent video case from "Batman Dark Night ARG" below.