What Does Radiosity Do?
Here there are actually two catchwords, radiosity and caustics. Radiosity refers to the realistic distribution of light through refraction and the dispersion of photons on objects. Light doesn't just consist of a ray that streams from the light source, encounters an object, and lights it up. With radiosity, light can be thrown back by the object on which it is shining or it can encounter another object that was previously not lit by the light source or one that lay entirely in shadow.
Existing shadows are lightened up and objects are lighted more softly and evenly. Highlights therefore look less hard. The rendered image gains a new, more realistic quality, for it behaves almost exactly as natural light ...