The Structure of the Data Dictionary

base tables	The foundation of the data dictionary is comprised of a set of base or underlying tables that store information about the associated database. Only Oracle should write and read these tables; users rarely access them directly because they are normalized, and most of the data is stored in a cryptic format.
user accessible views	The data dictionary contains user accessible views that summarize and conveniently display the information in the base tables of the dictionary. The views decode the information in the base tables into useful information, such as user or table names, and use joins and WHERE clauses to simplify the information. Most users are given access to the views rather than the base tables.