The MOO database operates in essentially the same manner as DOS, from the user's (that's you) perspective. The MOO database consists of a basic set of programs and functions. All other operations rely upon these programs and functions to operate. A computer which is being run with the MOO operating system is called the MOO server. 

"MOO" stands for MUD, Object Oriented, and a MUD is a Multi-User Dungeon (or Dimension)--a site that allows users to connect through telnet or other clients. MUDs offer users a text-based virtual reality and the ability to communicate in real time. 

Most MOO's have a basic theme around which they are developed and built. Diversity University is an educational moo, so the building and development has been that of a college campus. Another moo, called BayMOO, has a theme of San Fransisco, so all of the building and development there has been related to San Fransisco (past and present). LamdaMOO, the mother of all MOO's (the first moo database which serves as the foundation for most other MOO's), is purely social and it is developed around the theme of a fraternity house. There are literally dozens (hundreds?) of other moos, many listed at http://www.daedalus.com/net/moolist.html.