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Normal map is a raster image (texture), each pixel of which is a normal's vector. It is possible to say, that normal map carries information about relief in each its pixel.
The use of normal map only then makes sense when you need to have a good detailed image. All ready normal maps are used via UV-editor. Thereby some limitations appear concerning their use. For example, the necessity of having the same resolution for image texture and normal map. That is why, firstly, we should create image texture with necessary resolution, then create just the same normal map.
At the same time it is necessary to keep in mind that textures created in 3d editors as a rule store sufficient information about surface, that is why sometimes normal map may worsen your image making it excessively detailed. In this case you can use the same texture as Bump map.

Normal map can be created only from a raster image:
- UV-unwrapping, created in UV-editor and baked into normal map;
- ready 2d texture transformed into normal map by means of Gimp plugin or PhotoShop plugin.

When using ready textures for making normal map, the use of Gimp plugin is quite sufficient (Filters >> Map >> NormalMap). All you need to do is to press the "Ok" button, as default settings work correct. If you don't have this plugin installed, you can download it here: http://nifelheim.dyndns.org/%7ecocidius/normalmap/#download

It is necessary to note, that normal maps, as well as bump maps don't modify meshes; they create the illusion of a relief on an unchanged mesh, that is why the bigger camera angle the smaller effect.

To illustrate all, let's look at the following example:

In this example all images were rendered under the same shader settings, so that the difference between images could be noticeable, however, the difference between 2 and 3 images is practically indistinguishable, but having a thorough tuning of brightness and contrast parameters the difference between them can be nullified.

For making more detailed relief, other methods are used, such as Displace and other mesh modifiers, that require a big amount of polygons to be present. We'll talk about this in the next tutorial.

Best wishes, AssistCG