ARGs or Alternate Reality Games to give them their full title are quite difficult to explain to people. So instead to attempting to do it myself, I thought I would rely on those far more authoritative than myself.
Argology has the following definition of ARGs:
Alternate Reality Games (ARGs) tell stories through narrative elements that are distributed across various platforms. These game variables are carefully concealed from players until appropriate moments determined by the game designer(s). Game play involves players working collaboratively through email, phone/sms contact, real-time interactions and extensive online engagement. Players generally react to narrative cues that are projected across numerous forms of media. These include media technologies that are not traditionally associated with games that, unlike ARGs, rely on a single platform for communication (eg console games). In doing so, ARGs make players step outside the restrictions of mono-genre game boundaries.
Instead of requiring the player to enter a fictional game world, ARG designers attempt to enmesh the game within the fabric of the player’s real world by harnessing as many media technologies and interfaces as possible. By doing so, ARGs expand the frame for the game beyond the computer monitor or television screen, effectively making the entire world the “game board.”
Wikipedia defines them as follows:
An alternate reality game (ARG) is an interactive narrative that uses the real world as a platform, often involving multiple media and game elements, to tell a story that may be affected by participants’ ideas or actions.
The form is defined by intense player involvement with a story that takes place in real-time and evolves according to participants’ responses, and characters that are actively controlled by the game’s designers, as opposed to being controlled by artificial intelligence as in a computer or console video game. Players interact directly with characters in the game, solve plot-based challenges and puzzles, and often work together with 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.