The term MMO is an acronym for the phrase, Massively Multiplayer Online Game.They can also be known as MMORPGs or Massively Multiplayer Role Playing Games. As opposed to traditional computer or console games which have a limited capacity for multiplayer interaction, MMOs allow for a virtually unlimited number of players to interact in the same game. MMOs are played on the Internet and revolve around expansive virtual worlds hosted by massive servers.
MMOs are popular largely because they enable players to immerse themselves in a virtual environment in which they can explore their surroundings, as well as cooperate and/or compete with each other controlling their unique avatar. Another draw to MMOs, especially games like Second Life is the social aspect. Players can meet and form bonds with other players who have the same interests and goals as themselves.
A few important characteristics of MMOs are that they typically require players to spend enormous amounts of their time playing the game, and are infamous for being extremely addicting. Furthermore, most MMOs demand a monthly or yearly subscription fee, as a means of profit or to help maintain/develop the game. Some examples of MMOs are Second Life and World of Warcraft.
Fairly recently however, MMOs have been becoming more and more accessible to gamers who might not want to pay in order to play. An example of this is the MMO Guild Wars, which was released in April 2005. The user pays for the initial game but all costs beyond that are dedicated solely toward further expansion packs. Subscription to the game itself is free.