Someone asked:

Can someone please tell me what MUD is? (not dirt)

To which StarWolf replied:
MUD stands for Multi User Dungeons, or, Dimensions, whichever you prefer. Essentially they're like a combination of a game and a Chat channel. You adventure in a shared user text-based virtual reality where you read descriptions of rooms and objects and interact with other players who are logged in to the same computer..many of these are combat oriented where you are trying to survive while killing others..and many are socially oriented where you essentially just interact with the other players. After a period of time..and if you are daring and know even a little programming you can help build and create these worlds.

Michael Stelly describes it like so:
MUD's are Multi-User Dimensions. Real-time fantasy worlds allowing you to create a character based on that world (i.e. mages in a D&D world or Stormtroopers in a Star Wars world). You interact with other people's characters via the keyboard. Fortunately, there are plenty of typing shortcuts for those of us with less than dextrous fingers. Follow the alt.mud newsgroup for current MUD, MUSH, or MOO listings. Find one that appears interesting and telnet to that location. Then, once you're there, read the help files and be sure to let people know you're a "newbie". Above all, don't be shy about asking for help. A very "newbie-friendly" MUD is the MozartMUD. [listed below] Good Luck and Happy MUDding!

Jack Rhondeau, Jr. posted an even more elaborate description, which apparently is a modified file from yet another source:

----MUD----

A MUD (for Multi-User Dungeon) is an aetherial location where users engage in real-time fantasy play. This play includes conversation,but it also includes written descriptions of behavior. Users describe their actions, create imaginary objects, and design the MUD's architecture; all in typewritten words.

The MUD acronym is derived from the use of these electronic environments for role-playing fantasy games like Dungeons and Dragons. As MUD software has multiplied, so have the acronyms. You will find MUDS called MOOs (Multi-User Dungeon, Object-Oriented), as well as an assortment of MUSHes and MUCKs. MUSH stands for Multi-User Shared Hallucination. MUCK, which derives from MUD, stands for nothing in particular.

Individual MUDs develop unique cultures and histories, but the shared technical basis of MUDding gives rise to certain stylistic similarities. If you choose to explore the world of MUDs, your initial MUD will be rather difficult to learn. However, once you are comfortable with the logic of MUDs, you should be able to try new ones more easily.

MUDs are usually controlled by "wizards" or "gods" or "immortals". These are users with extraordinary powers. On most MUDs, the wizards are omnipotent; they can freeze your character and kick you off the MUD if they feel like it. If they use these powers benignly, MUD wizards, like BBS sysops, can help create a congenial and active electronic environment. If something in a MUD goes wrong, however, everybody enjoys blaming the wizards.

All MUDs operate a little differently. For a great introduction, we recommend reading the three basic MUD FAQs. These can be obtained via Anonymous FTP: Part 1, Part 2, and Part 3.
You can also retrieve them via Gopher from gopher.luth.se (follow the menu path: "FAQs" then "rec" then "games" then "mud)". They are also posted regularly in the Usenet newsgroups rec.games.mud.announce, rec.games.mud.misc, news.answers, and rec.answers.

To overgeneralize a bit, there seem to be four main types of MUDs: combat, fantasy/role-playing, social, and professional (or educational). In combat-oriented MUDs, characters compete with each other in elaborate scenarios of sword and sorcery. In fantasy/ role-playing MUDs, users adopt fictional personas but do not necessarily do battle. Role-playing MUDs are sometimes based on popular books or TV shows, such as Star Trek or Anne McCaffrey's Pern universe.

In social MUDs, the emphasis is on non-competitive interaction and communication. Fictional disguises are not always welcome. Finally, professional and educational MUDs offer an environment where fantasy and role-playing facilitate career and research goals.

If you are a MUD newcomer and don't have much experience in the world of fantasy games, you may want to avoid the combat-oriented MUDs at first. These tend to be larger, more difficult to master, and more evil in temperament.

How do you know if a MUD is a combat-oriented MUD or one of the other varieties? A MUD's character can often be guessed from the name of the MUD software. AberMUDs, DikuMUDS, and LPMUDs tend to be combat-oriented MUDs. Popular social MUDs include LambdaMOO, TinyMUSH, and TinyMUCK. However, the software type does not always determine the theme. To find out what is going on in a MUD, you may have to check out the action in the opening room, parlor, or closet. If you get asked a lot of questions dealing with strength and killing, you will have a pretty good idea of what to expect.

Recently, there has been an onslaught of newbies (newcomers to the Net), especially in the MOOs. This rush stems from the publicity MUDs have received in the popular press. Although relatively few aethernauts participate in MUDs, the highly emotional dramas of MUD land are fun to read about.

For a great example of a MUD in crisis, we suggest looking at Joshua Quittner's hilarious tale, "Johnny Manhattan Meets the Furry Muckers." Using Gopher, you can get it from gopher.wired.com. Navigate the menus "WIRED E-text" to "2.03 March '94" to "Features." Or, to obtain the story via email, send the message "get 2.03/features/muds" to infodroid@wired.com.

You might also check out the MUDInfo Link. This is a link to lots of information on MUDS, MOOS, MUSHES, etc. It contains a list of current active sites and a pretty comprehensive explanation of what each one covers.

If you want additional information, just access GOPHER and use VERONICA... :>)