Normal Mapping is the real-time way to add bumps to a surface. It is a lot like Bump Mapping, except the texture map you use to describe the surface detail is different. A Bump Map is normally a gray scale image where the bright spots are the peaks and the dark areas are the valleys on the surface. A normal map is a texture where every pixel is a normal vector, describing the Up direction of the surface at that point. A bump map can be converted into a normal map using the Normal Map TOP. There are other tools such as nvidia's Normal Map Plug-in for Photoshop Located Here that can also do this. Essentially, a normal map describes the normal for every pixel on the surface of a geometry, which is far more detail than you can get from point normals.
A normal is a vector in 3D space. The values of each component of the vector can range from -1 to 1. When looking at a normal map, up is +Y, down is -Y, left is -X and right is +X. +Z is coming out of the image and -Z is going into the image. So if you are looking at a bump on the image, the left side of the bump will have a negative X value, the bottom of the bump will have a -Y value and so on.
One catch however is that while normal vector values range from -1 to 1, the values in a 8-bit texture can only range from 0 to 1. Because of this, the normal vector values are offset in the normal map to fit in the 0-1 range. -1 becomes 0, 0 becomes 0.5 and 1 becomes 1. So for example a value in a normal map of (0.5, 0.5, 1.0) corresponds to a vector of (0,0,1). Similarly a value of (0.5, 1.0, 0) in the normal map corresponds to a vector of (0, 1, -1). The majority of normals on a surface will be pointing up off of the surface, so most of them will be close to (0.5, 0.5, 1). This is why normal maps always look blueish.
To use normal mapping, you need to create tangents (attribute T) on your geometry. This is done with the Attribute Create SOP.
For a more in-depth explanation of how normal maps and tangents are used to create the look of bumps on geometry, refer to the Tangent Space Normal Mapping article.