Normal mapping

In 3D computer graphics, normal mapping is an application of the technique known as bump mapping. Normal mapping is sometimes referred to as "Dot3 bump mapping". While bump mapping perturbs the existing normal (the way the surface is facing) of a model, normal mapping replaces the normal entirely. Like bump mapping, it is used to add details to shading without using more polygons. But where a bump map is usually calculated based on a single-channel (interpreted as grayscale) image, the source for the normals in normal mapping is usually a multichannel image (x, y and z channels) derived from a set of more detailed versions of the objects. The values of each channel usually represent the xyz coordinates of the normal in the point corresponding to that texel.
--wikipedia http://en.wikipedia.org/wiki/Normal_map