This document is hyperlinked to the generated Javadocs through the UML diagrams. Pending changes are documented on the PEGboard.
The overall division of Gzz is shown in the package diagram below.
The Core APIs define the Java interfaces to the zzStructure and Xanadu media model. The Backend implements these APIs. The FrontEnd is responsible for providing a user interface to the structure, using (if using the OpenGL version) the Gzz graphics library. The Overall control is simply the meeting place of the backend and the frontend: all code that touches both should be there. It's mostly about startup.
The frontend and backend are different packages: they do not imply anything about different processes.
Here is the same picture, slightly expanded.