Oracle Data Dictionary

Oracle Data Dictionary

The data dictionary is a repository of information about the Oracle database, known as metadata. Metadata is “information about information,” and the data dictionary is information about the database. There are hundreds of views in the data dictionary.  
Metadata is data about data, or data that defines other data.The Oracle data dictionary is metadata about the database. 
For example, if you create a table in Oracle, metadata about that table is stored in the data dictionary, such things as column names, length, and other attributes are stored. Thus, the data dictionary contains a great volume of useful information about your database. 
Pretty much everything you would want to know about your database is contained in the data dictionary in some form.

* dba_all_tables
* dba_indexes
* dba_ind_partitions
* dba_ind_subpartitions
* dba_object_tables
* dba_part_col_statistics
* dba_subpart_col_statistics
* dba_tables
* dba_tab_cols
* dba_tab_col_statistics
* dba_tab_partitions
* dba_tab_subpartitions

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 the user has permission to