Once you reach 30 posts you are automatically promoted to "Ranch hand"
Bartenders are forum moderators and are chosen by existing moderators and administrators - they have mod authority on selected forums.
Sheriff's are the board administrators who have admin rights on the whole forum. Sheriff's are normally promoted from the bartender ranks.
Other titles can be bought by donating $100 to JavaRanch.
Edit: Forgot one - some folks may have Author in the title - I think I read somewhere that the "Amazon" test is applied when deciding whether someone gets this title. [ August 23, 2005: Message edited by: Rick Beaver ]