Radiance is a set of tools which provide an advanced lighting simulation software package. It allows very detailed definition of materials' and light sources' properties through the use of advanced techniques like brdf and ies files, as well as a powerful mathematical scripting language, resulting in accurate renderings of scenes which are usually imported from CAD and 3D modelers. It uses the hybrid approach of Monte Carlo and deterministic ray tracing to achieve a reasonably accurate result in a reasonable time.
Radzilla is an advanced lighting simulation system. It is based on Radiance, but features additional extensions and a GUI. It is targeted mainly to Radiance users who are a bit more courageous about new features and optimizations and to those who would like to enjoy the power of Radiance but have been afraid of its pure command-line interface so far. Radzilla tries to stay as compatible as possible with "classic" Radiance. Even many tools and files from the classic distribution are used.