The Data Directory is what we call the internal tables and views that Oracle stores its own data in. The tables in the data dictionary are broken down into 3 different set of views: ALL_, DBA_, and USER_. The ALL_ views display all the information accessible to the current user. The DBA_ views display all relevant information in the entire database. The DBA_ views are only intended for admins. And finally, the USER_ views display all the information from the schema of the current user. So the big difference between the USER_ and ALL_ views is that you can only look around in your own schema using USER_ views but the ALL_ views can look at all the schema's the user has permission to. Because these are views, most of the data is duplicated between them, for instance: USERS_USERS, ALL_USERS, and DBA_USERS.
You can find a complete listing of all the data dictionary tables and views in the Oracle Database Reference manual: