Active data dictionary is a data dictionary managed by DBMS. Every change in database structure (using DDL - Data Definition Language) is automatically reflected in the data dictionary.
Accessing data dictionary
Most relational databases provide read access to its active data dictionary with a predefined set of read only tables or views that you can query to get access to database metadata - list of tables, columns, relationships etc.
Different vendors use different names for its data dictionary - system catalog, catalog tables, data dictionary, information schema, but the idea is almost always the same.
Here is a big database of useful queries to active data dictionary of various DBMS.