Educational - These MOOs are focused on providing a learning environment for all users.
ESL - English as a Second Language. These MOOs are conducted primarily in a language other than English.
Programming - These MOOs are focused on MOO coding and development in other behind-the-scenes coding languages.
Research - Similarly to educational MOOs, these MOOs are focused on learning new things, but in them MOO users are typically the subjects, rather than the students.
RPG - Role Play Game. RPG MOOs are focused on a fictional theme in which MOO users create a character and interact with other users in conjunction with the MOO's theme.
Social - The average MOO is a social MOO. Social MOOs are MOOs with the main purpose of interaction with other users, based on you being yourself, not another character.
Themed - Themed MOOs are simply MOOs with a theme. Both RPG and themed MOOs have a theme, but typically in a themed MOO the users need not correlate to the theme at all. Themed MOOs range from very strict in theme to very loose and casual, however, so be aware of the rules, guidelines and canon of a MOO before joining it.