A data dictionary is (generally) a read-only set of tables and views that holds metadata - used in a database management system. The data dictionary may be considered a database in its own right.
Amongst other things, a Data Dictionary holds the following information:
*Usernames, Roles and Privileges
*Stored procedures and Triggers
*General Database Structure
One benefit of a well prepared data dictionary is a consistency between data items across different tables.
several tables may hold telephone numbers, using a data dictionary the format of this telephone number field will be consistent.