Jeff Pack, Brown University '99 (English 112, 1996)
MUDs - short for Multi-User Dimensions - are basically the network equivalent of a text adventure, the major difference being that in a MUD there usually isn't a predefined story. It's just made up as the players go along. (Which, if done right, is as good as any prewritten work of interactive fiction; unfortunately, it more often degenerates into groups of avatars standing around talking - the MUD equivalent of an IRC (Internet Relay Chat - the Internet equivalent of one of AOL's chat roms).
Personally, I never really got into MUDs, though I've heard stories of college students so addicted to them they spent every waking hour connected. The attraction of text adventures lies in the quality of the writing, and regrettably most MUDs aren't populated with writers as gifted as some of those who produce stand-alone interactive fiction. Then again, maybe I've just been looking in the wrong places.