Specular Mapping
Is the specular equivalent of diffuse-mapping.
This is a simple way to change the reflectivity of parts of an objects surface. Consider a wooden box with a metal rim, the metal is reflective and should have a specular-lobe but the wood isn't and shouldn't.
We can get this affect by specifying the specular intensity through another texture. We multiply the specular value we get by the equivalent value at the same coordinates on this texture. This specular texture should be black (rgb = 0) at points where no highlighting should happen. This effectively cancels out the specular highlighting at that point.