A DDL trigger is a special type of trigger that fires in response to a Data Definition Language (DDL) event. It can be used to audit and control changes to the database schema, such as when a table is modified, or when a user attempts to create or drop a table.
For example, you could create a DDL trigger to log any changes to the database schema, by logging the SQL command that was executed, or by sending an email to the DBA. You could also create a DDL trigger to block certain users from creating or dropping tables, by raising an error when the command is attempted.