Multi-User Dimension (MUDs) started as interactive adventure games similar to Dungeons and Dragons for the computer--but a version many people could play at once. Since those days, the use of MUDs have expanded to other sorts of games and to more social uses. The object-orientation of MUD Object Oriented (MOOs) puts more of the programming focus on the "objects" that are in the MOO. In other words, it is like a real-time conferencing program that adds a pseudo-physical dimension: Players talk in rooms, can move between rooms and interact with objects.
MOOs have been extensively used by Virtual Classrooms projects, the two most well known are the GNA (Globewide Network Academy) and The Diversity University. They are the first attempt at having a text-based synchronous Virtual Classroom.