In my last post I gave examples of using both the pre-built reports and building custom reports on your database models.
As a P.S. to that post I want to point you to the JDeveloper Help system that comes with diagrams to help you define and understand the Database Reporting metamodel. Below is the JDEV table of contents - open at Working with Database Reports
and one small part of the Tables Report Metamodel to give you the idea. Have fun!