Massively multiplayer online game
Massively multiplayer online games, or MMOG's, are computer games in which many players are simultaneously inhabiting the same game environment run by one or more servers. All inputs by the player are considered "client side" inputs and are recorded by the server which then distributes this data to the other players.
A popular example of a MMOG would be "World of Warcraft" created by Blizzard Entertainment. In World of Warcraft, or WoW as players themselves call the game, thousands of people inhabit the same game environment embarking on quests to find epic items and to battle evil enemies. Players can cooperate with each other and help each other complete their own quests or can instead choose to battle each other. But all of this takes place within a game world.
Massively Multiplayer Games are not limited to fantasy environments. There are many genres of MMOG's including massively multiplayer racing games, economic games, government simulations, Pirate worlds, military simulations and more.