An alternate reality game (ARG) is an interactive networked narrative that uses the real world as a platform and employs transmedia storytelling to deliver a story that may be altered by players’ ideas or actions.
The form is defined by intense player involvement with a story that takes place in real time and evolves according to players’ responses. Subsequently, it is shaped by characters that are actively controlled by the game’s designers, as opposed to being controlled by an AI as in a computer or console video game. Players interact directly with characters in the game, solve plot-based challenges and puzzles, and collaborate as a community to analyze the story and coordinate real-life and online activities. ARGs generally use multimedia, such as telephones, email, and mail but rely on the Internet as the central binding medium.Wikipedia: Alternate reality game