MMORPG is a commonly used acronym for massive multiplayer online role playing game.
The most famous at present is World of Warcraft, an extremely popular (and lucrative) offering from Activision Blizzard. The business model is a monthly or longer subscription, approximately $15 of cash flow up-front per month. As of late July 2008, the game counts more than 11 million players. You do the math. Activision Blizzard is minting money, and Warcraft is a billion-dollar property (not even counting brand value, etc.).