The built-in database can be found in the deployment/data/database/hsqldb folder. It's a very simple database based on a text file that will be loaded into memory at the time of running the application. For more information I recommend you check out http://hsqldb.org/. If you want to delete the contents then shut down your application and simply remove the database folder altogether.
You can also view the contents of the database with the built-in database viewer (see the console in the Modeler)
If you would like to see the data entered into entities already in your database, you can build some simple forms with datagrids to display this information. Alternatively, there is a module in the App Store called Object Browser that should let you see entities in the database and objects within those entities.
Hope this helps.
The hsqldb database can be found in your deployment/data/database directory. There you will find the sql commands and the default.script. If you want to find out more about hsqldb go here