What is a MUD?
From Jargon File (4.3.0. 30 APR 2001):
MUD /muhd/ n. [acronym, Multi-User Dungeon; alt. Multi-User Dimension]
1. A class of virtual reality experiments accessible via the Internet.
These are real-time chat forums with structure; they have multiple
`locations' like an adventure game, and may include combat, traps,
puzzles, magic, a simple economic system, and the capability for
characters to build more structure onto the database that represents the
existing world.
2. vi. To play a MUD. The acronym MUD is often
lowercased and/or verbed; thus, one may speak of `going mudding', etc.