The very brief version: MUD is an abriviation of Multi User Domaine. A net-site where many people can be at the same time.
The definition of the MUD varies according to who is defining it and I have therefore chosen to provide the reader with 3 definitions which, if all read, gives a more complex picture of what people "outthere" think of as MUD.

The short version

A longer version

The professional version (??)

The MUD-connector is a website that lists almost all the MUDs on the net. Every MUD is listed and described according to a long list of parameters - and by browsing this site, you should get quite a good idea of the huge variety of MUDs that exist. Currently (June 2001), more than 1700 MUDs are listed here. When I first began monitoring this site less than 2 years ago, there were "only" about 1500 MUDs listed here. This, I think, goes to prove, that despite the low-tech design of these worlds (mostly text-based, not a lot of graphics), they are still thriving and not about to be overturned and left to die by the more fancy graphical MUDs of the 2D or 3D kind.

A special variety of the MUD is called the MOO. However, sometimes people use the words interchangably, depending on whether they use MUD in the broad sense (any domaine with many users) or a more narrow sense (a domain for many people with special characteristics).