A MUD or Multi-User Dungeon is an inventively structured social experience on the Internet, managed by a computer program and often involving a loosely organized context or theme, such as a rambling old castle with many rooms or a period in national history. Some MUDs are ongoing adventure games; others are educational in purpose; and others are simply social. MUDs existed prior to the World Wide Web, accessible through Telnet to a computer that hosted the MUD. Today, many MUDs can be accessed through a Web site and some are perhaps better known as "3-D worlds."
MUD participants adopt a character or avatar when they join or log in to a MUD. Typically, you can describe your avatar to the other participants. Each MUD has its own name, special character and ambience, and set of rules. MUDs are run by advanced participants or programmers called wizards.
Although many MUDs continue to be entirely text-based, some new MUDs use virtual reality settings and you can see the characters. However, the focus is on the exchange of text between participants who are logged in at a particular time. There are a number of variations on the MUD, including MOO s, MUCKs, and MUSHes, each associated with a server program of that name and varied mainly by the programming language used and the capabilities offered.