Data Dictionary Structure

Oracle Server : Consist of base tables and user-accessible views.

- Several views that summarized and display the information stored in the base tables of the  data dictionary.

- Most user cannot access to the base tables.

- The Oracle user SYS owns all base tables and user-accessible views of the data dictionary.

View naming convention :
- User : User's view (what is in your schema; what you own)
- ALL : Expanded user's view (what you can access like global..)
- DBA : Database administrator's view (what is in everyone's schemas)
- V$ : Performance-related data

Each view is distinguished by their prefix. For example, there is a view named USER_OBJECTS, another named ALL_OBJECTS, and a third named DBA_OBJECTS. These views contain similar information, but each of them has different scope. For views that are profixed with ALL or DBA, there is usually an additional column in the view named OWNER to identify who owns the object.
V$ holds information about performance and dynamic performance table is not a true table. Users can not access to this table, but they can access to a view that is created by an administrator.