From AionSource.com Wiki
Bump mapping is a computer graphics technique where at each pixel, a perturbation to the surface normal of the object being rendered is looked up in a height-map and applied before the illumination calculation is done. The result is a richer, more detailed surface representation that more closely resembles the details inherent in the natural world. Normal mapping is the most commonly used bump mapping technique, but there are other alternatives, such as parallax mapping.
See also here.
