RDO is an object interface that is closely tied to ODBC and is optimized for accessing SQL Server databases, especially from Visual Basic programs. It has little performance overhead on top of ODBC and has a relatively small memory footprint.
On the other hand, the DAO interface is closely tied to and optimized for the Jet local database, which ships in Visual Basic and Microsoft Access. However, for applications that access only SQL Server data, DAO offers less functionality and has a larger memory footprint. In such cases, it is not the recommended interface.
Remember that ADO has now become a widely used superset of most of the unique functionality and supported data sources of both of these interfaces.