one good thing about Nikon SLRs is that they share the same mount, the F mount. This means that you can use manual focus lenses on autofocus bodies and viceversa, (the exeption being the very early lenses, but they can be modified by a specialist).
"D" is just the name given to some autofocus lenses that have a chip which measures the distance the lens has focused on to calculate the flash output, (so this is only usefull for flash photography). Newer autofocus lenses also have this chip but they are no longer named D.
Manual focus lenses can be used on the latest bodies (film & digital) but the camera will only meter on manual mode. To get the full advantage of the camera's metering modes you can have these lenses chipped. This, I guess, is what Nikon means by "D-spec manual focus lenses".
For more info on Nikon, try this links: