Data Dictionary

Data dictionary is data(meta-data) for a database. The structure, relationship and usage of schema objects are maintained at data dictionary.
The implementation and feature of data dictionary is different by database products.

Oracle

Oracle maintains tables and views for data dictionary under SYS user(schema). Plain users can access views according to their roles and privileges.
User accessible views are named with prefix of 'USER_', 'ALL_', or 'DBA_'.

Oracle also maintain data or statistics of database's activity and performance in so-called dynamic performance views. Actually they are views named with prefix of 'V_$' and also owned by SYS schema.
Oracle provides public synonyms named with prefix of 'V$' for users properly privileged.

References

• Data Dictionary and Dynamic Performance Views
• Static Data Dictionary Views

DB2

DB2 UDB provides data dictionary under SYSCAT schema. SYSCAT.TABLES, SYSCAT.VIEWS, SYSCAT.COLUMNS, SYSCAT.INDEXES, SYSCAT.TRIGGERS are views describing the structure of database instance.

For dynamic behavior of database, DB2 UDB provides views under SYSSTAT schema.

References

• DB2 System Catalog Views: Everything You Need to Know About Your DB Objects