A multi-user dimension (also sometimes called multi-user dungeon, but in either case abbreviated to "MUD") is sort of a combination between the online chatting abilities provided by something like IRC and a role-playing game. A MUD built with object oriented principles in mind is called a "Multi-user dimension object-oriented", or MOO. Yet another variant is called a "multi-user shell", or MUSH. Still other variants are called multi-user role-playing environments (MURPE) and multi-user environments (MUSE). There are probably more. In all cases the differences will be mostly academic to the regular user, as the same software is used to connect to all of them. Software to connect to MUDs can be found for most platforms, and there are even Java based ones that can run from within a browser.