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The Data Dictionary

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: