1. Used to specify a database scheme as a set of definitions expressed in a DDL

2. DDL statements are compiled, resulting in a set of tables stored in a special le called a data dictionary or data directory.

3. The data directory contains metadata (data about data)

4. The storage structure and access methods used by the database system are speci ed by a set of definitions in a special type of DDL called a data storage and definition language.

5. basic idea: hide implementation details of the database schemes from the users.