To quote the Mud Faq hosted on the Mud Connector:

"A MUD (Multiple User Dimension, Multiple User Dungeon, or Multiple User Dialogue) is a computer program which users can log into and explore. Each user takes control of a computerized persona/avatar/incarnation/character. You can walk around, chat with other characters, explore dangerous monster-infested areas, solve puzzles, and even create your very own rooms, descriptions and items."

For more information on MUDs and MUDding, see the Mud Faq hosted by the Mud Connector.

So why are you telling me this?

Because I want to introduce you to the MUDs I have known and loved, and they'll mean more to you if you have an inkling what exactly a MUD is.

And the MUDs you love are...?

Discworld Mud