Multi User Dungeons are role playing games on the Internet. You can decide to play on adventure based MUDs, fight based MUDs and others. These virtual worlds only do exist in a computer, but there are many people playing and sometimes 'living' in these worlds.

It is a text based adventure and it really is great. Absolutely fascinating and you might get into addiction, if you once start playing these games. (Or you might hate them)
Nils (aka Aldus@unitopia.de) and I (aka Graham@unitopia.de) sometimes did play in UNITopia, which is a german MUD.

During the time we've been playing that MUD we've prefered Kmud as MUD-Client, which you might want to take to give these games a try.

If you get into some trouble compiling Kmud 1.0 on a newer KDE system, then check out the Kmud 1.0 compile-HowTo, which I've written. Actually you can get an german and italian translation of this HowTo from the homepage of Kmud.

Further more you might get interested in setting up your own MUD at home. I've set up a MUD at home some time ago, using the UNItopia Libraries. The basic steps to get this HomeMUD up and running can be found in the HomeMUD-HowTo.