Many of those who will read this are probably already confused. "What is a MUD? What is Discworld?" The answers, my friend, will follow shortly.
First on the list: MUDs.

MUD, first of all, is short for Multi User D*. The 'D' can be one of many things. 'Dungeon' ranks at the top, followed closely by 'dimension'. What it is, is sort of a hybrid of IRC and AD&D. Like in AD&D, you can kill monsters, or help out the various NPCs (non-playing character, i.e. a monster, that's controlled by the MUD, and not a human), or just explore the world, or you can sit and talk with other people. Which is where the IRC part comes in.
That seems to handle MUDs, on to Discworld!

Discworld is a series of novels by the British author Terry Pratchett, or 'Pterry', as his fans have nicknamed him. It is, if I may be so bold, the fantasy alternative to The Hitchhiker's Guide to the Galaxy.
Discworld itself is a disc, resting on the backs of four elephants, who in turn stand atop a giant space turtle, the great A'tuin. Rather then get in depth about the plot and so on of Discworld, I can only suggest that you buy one of the books. 'The Colour of Magic' is the first in the series.
That handles Discworld, on to the Discworld MUD!

Reading the Discworld books is not a prerequisite to playing or enjoying the MUD. Nor, indeed, is even liking to read. The only real thing you need is a sense of humour and a willingness to learn. The players are quirky at best, and downright mind bogglingly weird at worst. The creators (those who run the MUD, making rooms, catching cheaters, etc.) are legendary for their odd and sometimes frighteningly sexual brand of humour.
I'm not sure what else to say, really, so I guess I'll close with some quotes from the players and creators.

Note: These quotes were gathered from various webpages made by player and creator alike, many of which can be found by using one of the search engines around.

(thieves) Illusori: heh, The perversity of nature is nowhere better demonstrated by the fact that, when exposed to the same atmosphere, bread becomes hard while crackers become soft.

(one) Saffra: And have some chicken soup, it'll make you feel better.
(one) Saffra: 2000 years of Jews can't be wrong.

Drakkos wisps: Two fish in a tank... one fish says to the other, 'Do you know how to drive this thing?'

Curious: Men like to get flowers too, for any women that are listening.
Curious: They give us something to eat between meals! Arr numm numm numm numm!

(cre) Dek's car caught fire last night while he was asleep in bed.
(cre) Terano: Do you know why?
(cre) Dek: Police don't know yet.
(cre) Dek: Bah. Bloody phone.
(cre) Eight: that's on fire too?

The address for Discworld is discworld.imaginary.com and the port is 4242. Use a telnet client to connect. For more information on the MUD, direct a browser to discworld.imaginary.com:5678. For more information on the Discworld series, visit www.lspace.org.