Since modern graphics hardware is able to render to multiple targets, the rendering process can be splitted into more than one pass. Deferre d shading is a name for the idea to split the rendering process into 2 (or more) passes.
A first pass is needed for the pixelwise calculation of the geometry, where all the information to shade the geometry is calculated and stored into buffers (render targets) on a per pixel level (normals, positions, unlit texture color, et cetera).
The actual shading is done in a second pass (or more, one for each light source in the scene), where the buffers can be read and the shading from each light source be calculated from them.
This way, it is possible to separate the material properties completely from the properties of the light, and account for them in different rendering passes. This reduces shader complexity, since if we want to add another light source type, we just have to write another lighting shader, and dont have to touch the material shaders.
These shaders were authored in AMD Rendermonkey, which I used to get a first hold on HLSL. The Rendermonkey project file is available for download here.
