A computer graphics effect that involves an image and a corresponding height map.
For each pixel in the image, a corresponding surface normal is calculated. Then, based on the location of the light source and angle of the surface normal, the pixel is shaded.
The result is a "three-dimensional" effect on the image, with shadows.
There are many ways to implement the algorithm; for example, for real time, quick shadings, the algorithm can be simplified (yes, this has been seen in many C64 demos =), with lookup tables, for example.
A GIMP plugin also exist for this effect; it is one of the Holy Plugins Everyone Must Master to make cool graphics.