It's a system to support the organizational mind. MUDs were created in the late 1970s to support interactive adventure games. Participants built their own electronic worlds, adopted new identities, searched for treasure, or fought wars. As MUDs got more elaborate, players used them to write software to make their games more exciting. MUDs became a programming tool.
MOOs are a subset of MUDs.
Curtis himself is best-known as the creator of LambdaMOO, which he unveiled in January 1991. LambdaMOO is a virtual world inhabited mainly by college students. The participants play games, discuss homework, and interact in ways that students everywhere interact. LambdaMoo is an evolving community, albeit one built on hundreds of thousands of lines of computer code, most of it written by its members. They engage people in a very active way.