Concerning MUDs in general, you can find many definitions on the Internet, a simple one that I found is:
"MUD - Acronym for Multi-User Domain, Multi-User Dimension, or Multi-User Dungeon. An online interactive computer game or exploration medium. Early MUDs were Dungeons & Dragons-like games in text-only forms, but they have grown into many other forms, too. They are now used for social gatherings, interactive learning, chat rooms, and much more, as well as the old-style entertaining games. Originally accessible only through telnet, many of them have migrated to the web, and even appeared in graphical virtual reality and 3D forms. Variations are known as MUSHes, MUSEs, and MOOs. For specific examples, see Delphi's selective DRAGONet Multi-User page and for more links, see Yahoo's list of links to MUDs, MUSHes, MOOs, etc. Good gaming!" (http://www.monster-isp.com/glossary/MUD.html)
Other definitions can be found for instance here:
fficial&oi=defmore&q=define:mud" rel="nofollow" >www.google.it]
The last one contains also some historical notes.
Albion MUD, the one I have been playing at, is a quite simple one, but big enough (in terms of its virtual space, rooms, areas atc.) and nice missions to be accomplished. It used to have quite a lot of players, right now, there not too many players, but it still is very friendly.
You can find more detailed and coloured information on its official web page: [www.veda.is
(there is also a link to connect to the game)
Personally I met some friends in that MUD from various parts of the world, and I am still in touch with them, and I also hada chance to experience teaching and learning on line how to code LPC which is the coding language of the MUD, an object oriented language derived from C, which existed before C+.
If you like Dangouns & Dragons, or similar games, or fantasy stories like The Lord of The Rings, this can be a nice place were you play and make friends and kill dragons, and other monsters. If you like also games with no graphics, you will enjoy it
In case you don't have a confortable telnet client, just download a simple but useful one, putty.exe, that you can find here: [www.chiark.greenend.org.uk
If you are lucky to use linux, just type "telnet mud.veda.is 4000" in your shell!