wxWidgets framework architecture

wxWidgets application framework

wxwin

High level languages using wxWidgets API

wxUniversal application framework

The wxUniversal is a project to create a set of widgets that are implemented in wxWidgets itself. In all previous ports, with some exceptions, each widget class is a wrapper around an existing implementation on that platform.
When you use wxUniversal, you have the same API classes set as wxWidgets, but these make a call to a custom widget drawing using GDI primitives instead of system ones.