Once you have used SQL to create a schema, the RDBMS stores information about the database and its tables in a series of files called the data dictionary^[1] (also called a catalog).

1. The data dictionary is the foundation of the database, it is where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns.
2. catalog: Also called the data dictionary. The catalog is the foundation of the database, it is where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns.

The data dictionary is the foundation of the database--it's where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns. If you modify a table, such as by adding a column or constraint, the RDBMS automatically updates the data dictionary to reflect the change.
Formats for data dictionaries differ among RDBMSs, but they usually contain:

1. Definitions of table columns
2. Referential-integrity constraints among tables
3. Permissions
4. Definitions of views and custom data domains

When a user asks the RDBMS to work with database data, the system looks in the data dictionary to find the table and columns the user requested. If they exist, the RDBMS makes sure the user has permission to work with the data and then writes it to the screen via a virtual table. Since the data dictionary contains referential-integrity constraints, the RDBMS can also ensure that the proposed change or update follows those constraints.
The next lesson describes the tables that are part of the data dictionary.

[1]data dictionary: The cross-reference of definitions and specifications for data categories and their relationships.