Lets start by mentioning what the radiance website says about Radiance:

Radiance is a suite of programs for the analysis and visualization of lighting in design.

Input files specify the scene geometry, materials, luminaires,time, date and sky conditions (for daylight calculations). Calculated values include spectral radiance (ie. luminance + color), irradiance (illuminance + color) and glare indices. Simulation results may be displayed as color images, numerical values and contour plots.

The primary advantage of Radiance over simpler lighting calculation and rendering tools is that there are few limitations on the geometry or the materials that may be simulated. Radiance is used by architects and engineers to predict illumination, visual quality and appearance of innovative design spaces, and by researchers to evaluate new lighting and daylighting technologies.

In short, we may say that Radiance is a suite of programs developed for performing lighting calculations reliably and with very few limitations. Its problem is that "it is a suite of programs" and not "a software". That is, it does not have any intuitive and nice Graphical Interface that guides you through the process of developing a model. In fact, using Radiance is quite frightening for some people. The role of Groundhog here is to transform this "suite of programs" into some sort of "software", that is easier and more intuitive to use by everyone.

RADIANCE binaries shipped with Groundhog are a courtesy of the National Renewable Energy Laboratories (NREL)