Dave Walvoord gave us a master course in lighting recently. He went over the various types and said to render each one by itself to make sure it looks good by itself. Here it is in action for shot 040.
Edge Light: This brightens up the hard edges and sculpts our subject a little better. In this shot, it's physically motivated by the screen.
Fill Light: The soft light that fills in those ugly hard shadows. Here, it's originating from the left of the camera, but adjusted to look as if it's coming from the computer keys.