Or "Everything is a library."
While the Hello World tutorial is geared towards getting you started with your own project, this page walks you through the basic structure of the qooxdoo SDK itself.
There is a page that gives you an overview of the physical structure of the SDK. As you can see there the SDK has four main components represented through the subdirectories application, component, framework and tool. Three of them, application, component and framework contain (either directly or in further subdirectories) qooxdoo applications or libraries that follow the general scheme for a qooxdoo application. In each you will find a Manifest.json file which signifies the adherance to the skeleton scheme. They also all contain a generate.py script which offers all or a subset of the standard qooxdoo jobs that you can run on a library, like source, build, test or api.
The fourth component, tool, comprises the tool chain and its various parts. You shouldn't need to worry about that since you interact with the tool chain through the generate.py script or one of the tool/bin scripts like create-application.py.
In the SDK's root directory there is - besides readme.txt and license.txt - an index.html that gives you an overview over and access to most of the SDK's applications and components. Just be aware (as mentioned on that page) that all of them need a generate.py build first in their respective directories. Only the Apiviewer for the framework is shipped pre-built with the SDK currently and can be invoked immediately.