Massively multiplayer online roleplaying game

Massively multiplayer online role-playing game (MMORPG) is a genre of role-playing video games in which a very large number of players interact with one another within a virtual game world.

As in all RPGs, players assume the role of a character (often in a fantasy world) and take control over many of that character's actions. MMORPGs are distinguished from single-player or small multi-player RPGs by the number of players, and by the game's persistent world (usually hosted by the game's publisher), which continues to exist and evolve while the player is offline and away from the game.

Additional Information
Wikipedia article