Psul aul , Jul 17, 2007; 10:31 p.m.
AF-S and AF-D are two completely different things. Many Nikon lenses are both AFS and D.
The D means the lens can report focus distance information to the camera, which the camera can then use in determining exposure (especially flash exposure). I think most current Nikon autofocus lenses are D lenses.
AFS mean the lens is equipped with a silent wave motor for fast autofocus operation. Nikon lenses that are not AFS are focused by a motor in the camera body. (I understand the D40 lacks this motor and can only use AFS lenses) AFS lens focus faster and quieter that non AFS lens. They are more expensive, but they are usually top end pieces of glass.
Internal focus is a whole other feature. An internal focus lens is designed so that it does not change lenght, and the front lens element does not rotate during focusing. Again, this a feature usually found on high end and some midrange lenses that many or may not be AFS.