Environment Lighting is a new feature introduced with the Blender v2.5 lineup. The algorithm uses light from the environment to light the scene in addition to any lights we may have.
In addition to the Energy value, which controls how much environment light Blender will take into account, there are three sources we can tell Blender to look at for light information. They are:
White, of course, tells Blender to assume that all environment light has a white color. This will result in a render remarkably similar to a scene rendered with ambient occlusion.
The following scene illustrates a raytraced Environment ...