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D would be a lens for full frame (FX) cameras. D800, d700, etc
DX would be a lens meant for a cropped sensor body. D90, d300, etc
You can use d glass on a crop body but it has to have the motor in the body to use AF. D7000 iirc this not an issue. D40...no motor in body so no AF for the other end.
Edit: If you use full frame glass on a crop body you jsut get a different perspecitve as it were. easy example. Crop sensor math go by 1.5 to estiamte how it will look.
A 50 1.4 D on a crop body will be more of a 75mm lens.