MUD or Multi-User Dimension
Buckle on your sword, grab your shield and prepare yourself for adventure in a text-based virtual world full of danger and intrigue. Quest with friends, or make new ones, and explore the realm, but be prepared to defend yourself! There is no charge to play.
MOO or MUD Object Oriented
Visit text-based virtual worlds and interact with Internet users from all the corners of the globe. Chat, explore, and meet new friends. There is no charge to play.
MUSH or Multi-User Shared Hallucination
Text-based virtual worlds or roleplaying environments, similar to MUDs, but without extensive competition and combat. There is no charge to play.
MUCK
These are taken from the concept "mucking around" or wasting time. These text-based virtual worlds have a high level of social interaction, similar to MOOs, but with roleplaying and a more defined theme.There is no charge to play.