I think of it as a topographical map with 255 layers, black is sea level, grey shade 1 = 1 step up, grey scale 2 = 1 step higher, and so on up to the highest mountain which is white. If you wish to create a good bump/displacement map you need to look at the surface texture and create a map of that. Some of my darker veins aren't raised at all. If you look at your hand the skin colour information has very little to do with the skin texture, which is why I called greyscaling the colour map a quick and dirty method. As out lined above the bump or displacement maps don't use colour data, they are a greyscale map with the black = none through to white = full, stepped up through the shades of grey information onto the object. So one set of files contains the colour maps and the other contains the texture. If it's silk it will have a fine texture, sacking will be course, regardless of colour. For instance a piece of cloth, may have a plain colour or pattern printed on it, but that is not the texture of the cloth.
They can be used as quick and dirty maps to give texture but they may be completely wrong. They don't have any texture information they are in fact colour maps. I think a part of the problem is that the files called 'texture' maps give a false idea.
