From Encyc

A MMORPG is a Massively Multiplayer Online Roleplaying Game. They differ from MUDs in that they have graphics, usually 3D graphics, and you use a mouse to control your character (or arrow keys etc). The term is meant to indicate that they can have many times more players online at a time; however MUDs can accommodate just as many players (although in practice no MUD has ever had more than a few thousand online at the same time). In other respects, MMORPGs and MUDs are the same.