A mud is a text-based Online-RPG, so it uses the text-based interface of the telnet-protocol that you can log in, type commands, read descriptions, what is happening, what other users say or do. You might have a look at
http://www.mudconnect.com/ , there you can read a bit more about MUDs and get a list of the MUDs available, what the setting is (future/past/fantasy/etc), if they are PK-MUDs or Non-PK-MUDs (PK: Player-Killing, you can kill or be killed by other players. Is absolutely forbidden in Non-PK-MUDs), and so on.
