Normal mapping is the process of using a texture to vary the surface normal of an object. It can be used to add details to a lower-poly mesh from a generated texture, or by using data baked from sculpting or displacement.
Normal maps are loaded via the image map node and by enabling the Normal map option. The node can then be connected to any material bump slot. The normal map bump height can be scaled with the Height setting. A height of 1.0 would be the default height of the texture.
Unlike regular bump maps the scale of a normal map is relative. For instance, 0.5 is half the bump of the texture. Thus you'll mostly want to to leave the height setting at 1.0 for normal maps. As with any image map, the object must have UV coordinates for using a normal map.
Mixing With Other Textures
The normalmap texture acts a special float(grey-scale) texture, and can be mixed with other float textures using the math node to produce complex bump maps.