This page is named the dungeon as a tribute to the first and most famous base for this kind of games: Dungeons and Dragons. Today still the majority of on-line games follow, more or less closely, the rules set out by that game.
It all began with DikuMUD. Well not exactly but it's close enough to the truth to be stated like this. Close to all muds of all flavours available still credit the original creators of DikuMUD in their opening screen.
MUD stands for Multi User Dimension (or Dungeon), a term that has gotten so popular that all games of this type are named MU*s. The dungeon deals with those games in particular, and only mentions the other multi-user games that did not originate from DikuMUD as an aside.

A very quick explanation of what the typical mud is like, as most of those who visit this page no doubt already are enlightened on this subject. On a mud you control a character that moves from room to room. Each room is represented by a textual description. In those rooms you might find other players, pieces of equipment (or junk) and often monsters. Players are expected to fight the monsters in the game, and as they do so they gain experience.
By earning a certain amount of experience players gain a new level and can learn new skills or magical spells, and usually they gain a little toughness and magical ability as well. This helps to fight tougher monsters, for more experience, and better equipment. The final aim of the game is to become so powerfull that you are deemed immortal, or at least a hero.
Most games have evolved quite a way from this but the basics are usually the same. The original DikuMUD had 4 different classes (or professions): Warrior, Thief, Mage and Cleric. Each had their strengths and weaknesses, and near all muds today still have at least those 4 classes available as well as numerous other refinements.

On this page there's little to be found about actually playing muds. The focus is on roleplaying on these games.

• Rhonda Peter's Guide to Roleplaying. This page details things that help to establish a character on a roleplaying game, and how to behave on those games.
• Roleplaying MUDs: pointers and discussion of various games that encourage, or enforce roleplaying by the players.
• Mud worlds: talk about and examples of designing worlds for muds.
• The !Overlord project: An attempt to design a game based around other things than fighting monsters.
• Stuck in the mud: Some usefull general hints and tips about muds in general and particular.