A MUD is a "program" to which a number of participants can connect through a network, and which permits each participant access to a data base of rooms, entrances and exits, and various objects.
Each participant controls the use of these data bases from within and "views" the objects that "exist" in the room in which s/he is visiting.
Each participant can add new objects to the database, and can describe the attributes of the objects which s/he creates.
The interaction between the participants in the MUD is a sort of conversation. Each participant can choose a name, his/her gender, and physical appearance.
This choice of name, of gender and of physical appearance permits, on the one hand, the realization of fantasies, while on the other it raises questions as to the reliability of the information that participants give and receive.
These characteristics permit the participants a large degree of anonimity, and the security to express thoughts and feelings that are even extreme - thoughts that they might not dare to express in "real life".
From an educational perspective we should consider the "wizards" - creators of the MUD and/or experts in its ways, who sometimes find themselves called upon to act as the police or the law, establishing elementary behavioral groundrules in what might otherwise be an anarchic situation.
Conversations in a MUD can be a reflection of what is happening in "real life", and after a while a distinct community and identity come into being, exemplified by a consensus on the language, codes of conduct, social rules and conventions, and public areas.