A massively multiplayer online game (also called MMOG or simply MMO) is a video game which is capable of supporting hundreds or thousands of players simultaneously. By necessity, they are played on the Internet, and feature at least one persistent world. They are, however, not necessarily games played on personal computers. Most of the newer game consoles, including the Xbox 360, PlayStation Portable, PlayStation 3, Nintendo DS and Wii can access the Internet and thus can have MMO genre games.
MMOGs can enable players to cooperate and compete with each other on a large scale, and sometimes to interact meaningfully with people around the world. They include a variety of gameplay types, representing many video game genres.
Source: http://en.wikipedia.org/wiki/Massively_multiplayer_online_game
