You still don't understand? Bump is used to add height detail to a flat surface. It changes how lighting falls on it to alter the surface slightly giving the illusion of depth. Now its called normal map and its far more advanced but its the same idea. The diffrent settings will most likely be loading in the normal map texture at diffrent resolutions for more or less detail/texture memory usage.
As others have demostrated, it lightens the edge that is facing the light and darkens the edge facing away. That crates highlight and shadow. Thats all that is nessasary for your eyes to belive its a raised surface.
I still prefer the prego idea though...