VCL, or Visual Components Library is the responsible module for widgets (windowing, buttons, controls..) operating system abstraction, including basic rendering (e.g. the output device).
This page gathers information about the VCL module. Since the Mac OS X team is currently investigating on Aqua UI, we will try to document everything of interest on this page and its subpages. More to follow...
To complete, you can find another effort (relative to native Mac OS X port) with more informations about how VCL module works there : Work in progress