The data definition language (DDL) is a language by which we could state a set of definitions used to define database schemes. The end result of a compilation of DDL statements is a pair of tables and these tables are kept in a special file called data dictionary or dictionary.
The dictionary comprises”data about the data” which is sometimes called metadata. So data dictionary (or data repository) is defined as a centralized repository of data, including design decisions, usage standards, application program descriptions, and user info. From the dictionary, the various schemes, outside and conceptual mappings are physically stored in the source and object form.
The data dictionary is utilized to signify more general software usefulness than a catalog. A catalog is closely combined with the DBMS software and it’s mainly accessed by the various software versions of the DBMS itself, including DDL and DML compilers, etc.. The software packages to get a standalone data dictionary might interact with the software modules of DBMS, however, it’s mainly used by designers, users and DBA of a computer system. When a data dictionary system is used solely by the designers, users and DBA (not by the DBMS applications ), it is referred to as an inactive data dictionary, otherwise, it is called an active dictionary or data dictionary.