I could argue that all database management systems (DBMS) have a thing called data dictionary. It is a set of views or tables that provide access to metadata about database schema - list of tables, columns, keys, etc. They are located in special system schema or are available implicitly in all schemas.
Standard SQL data dictionary
SQL-92 standard (for relational / SQL databases) specifies data dictionary called information schema. This standard defines tables data dictionary provides ..
Proprietary data dictionary
Most popular databases provide also their proprietary data dictionary, often referred to as System Catalog (or system views, system tables or simply catalog).
Typical data in DBMS data dictionary
- Foreign keys
- User defined functions
- Stored procedures
- User groups
- Size of tables and indexes in KBs
- Number of rows in tables
- Connection history
- Query history