RDBMS = Relational Database Management System. This is the traditional relational database.
ORDBMS = Object-Relational Database Management System. This is an RDBMS that also supports storing user-defined objects (types) in the database. So you might declare a type "POINT" that has x and y components, and then create a table with a column of type POINT. The type may also support "methods" (functions) like "distance_from_origin".