A bump map simulates a bumpy or irregular surface.
You can select an image file or procedural maps to use for mapping. Bump mapping makes an object appear to have a bumpy or irregular surface. When you render an object with a bump-mapped material, lighter (whiter) areas of the map appear to be raised and darker (blacker) areas appear to be low. If the image is in color, the gray-scale value of each color is used. Bump mapping increases rendering time significantly but adds to the realism.
Use bump maps when you want to take the smoothness off a surface, or to create an embossed look. Keep in mind, however, that the depth effect of a bump map is limited because it does not affect the profile of the object and cannot be self-shadowing. If you want extreme depth in a surface, you should use modeling techniques instead. The bumps are a simulation created by perturbing face normals before the object is rendered. Because of this, bumps do not appear on the silhouette of bump-mapped objects.