i dont have an answer for your first question, but if i understood your second question right, you want to know the uses of each type of map.
Normal Map - Normal maps are used to add details to your model without changing the actual geometry, like adding scales to a reptile or tiny bumps on skin. To apply a normal map in Blender, first go to the texture panel of the material that you will use. once added a texture, changed it to an image type, and retrieved the normal map image from your computer, look for the “Image Sampling” tab still under the texture panel, then maximize it. In the “Image Sampling” box, check the “Normal Map” checkbox, and make sure tangent is selected in the dropdown below it. Checking that box tells blender that the image your using is a normal map, so that it can handle it appropriately. Now, open the “Influence” tab. under that tab, uncheck ‘intensity’ under Diffuse, because we do not want blender to take the color from the normal map. Check the “Normal” checkbox, and make sure the influence slider beside it is set to one. (You can increase or decrease the value to get more dramatic/subtle results). If your using one of the newer versions of blender, at the very bottom of the “influence” tab, there will be a “Bump Mapping Method” dropdown that you can change from ‘default’ to ‘best quality’ for better results. Now in the preview window, you should see the details from the normal map on the preview mesh. Also, make sure that your image mapping is the way that you want it. If i havent forgotten anything and everything went right, that should be all 
Displacement Map - A displacement map is similar to a normal map, only it DOES affect the actual geometry of the mesh. I believe there black and white, unlike normal maps which are colored. If you want to use a displacement map on your model, your model has to have dense faces, otherwise you won’t get a lot of detail. the difference between a normal map and displacement map is that a normal map is only faking the details, whereas displacement maps are playing with the actual faces to get the details. use normal maps for small details like reptile scales or engraved text, but use displacement maps for large details like bricks on a house. to use a displacement map, again, select the image you want as the displacement map, and under the “Influence” tab, check the “Displacement” checkbox (second one under normal checkbox) and set the value to one. now when you render, you should see the detail, but only if you have a high number of faces.
Specular Map - If you look at different parts of your skin, you’ll notice that some parts are glossier than others. in blender, the shiny parts have higher specular intensity than in less shiny parts. a specular map uses black and white colors, lighter colors telling blender to make that part glossier, and darker colors to make that part duller. these can be used just about everywhere. to set a specular map up, you need to upload your spec map, and again under the “influence” tab, uncheck ‘intensity’ under diffuse and check “intensity” under the ‘Specular’ section. now, wherever its lighter, its shinier.
Diffuse Map - Diffuse maps are simple and most common. diffuse = color. its the actual colors of the texture. not much to them. set them in blender by loading them and making sure that there ‘intensity’ value under Diffuse is set to one. thats all (besides mapping).
that’s a description from what i know. hoped it helped