This is the source documentation of FreeCAD. It is automatically generated from the source code, and describes the different components of the FreeCAD source code, for both the parts written in C++ and Python.
For general help and documentation about the FreeCAD application and how to use it, head first to the Wiki Documentation.
Refer to the general introduction to the FreeCAD source code page for a broad view of how the source code is organized, and browse the modules, classes and namespaces from the menu above.
The Developers section of the wiki also contains additional information for developers, and the Powerusers / python scripting section contains coding documentation that is specifically aimed at python scripting, but that will also be useful to C++ coders, as most of the APIs are identical or very similar.
For space reasons, this on-line version doesn't include the source code headers nor the full inheritance graphics. But you can build a full version yourself with everything included, as explained in building the source code documentation. You can also browse through the source code on github.