Nikon Lens
Nikon lenses are coded with letters and numbers indicating the properties of each lens. Two of the letters that help define types of lenses are D and G. They represent two different aspects of a lens' attributes.
"D" Type Lens
In 1992 Nikon began producing "D" lenses. A Nikon lens with a "D" in its name refers to the lens' ability to communicate distance to Nikon camera bodies. The camera body can make adjustments to exposure and flash settings based on this distance, resulting in better overall exposures. Most new Nikon lenses are "D" lenses and may have "AF-D" in their title. Some lenses have the "D" tag after the aperture information. An example is the Nikon 105mm f/2.8D. All "AF-S" and "G" lenses have the "D" feature although it is not listed in the lenses' titles.
"G" Type Lens
Nikon began making their "G" lenses in 2000. The "G" refers to the lack of an aperture control ring on the lens itself. The aperture is the opening inside the lens where light enters the camera. Aperture control refers to changing the size of that opening, making it larger or smaller. The aperture setting for the "G" type lens is controlled by a dial on the body of the camera or by the camera itself in auto exposure. A lens title example bearing the "G" is AF-S Nikkor14-24mm f/2.8G ED. The "G" indicates that the aperture will be controlled through the body of the camera and not on the lens itself.
The difference
All new Nikon auto focus lenses incorporate the "D" type feature even if they don't bear the tag. Most also are "G" type. So the difference is not between the "D" type and "G" type; it's between the "G" and non-G type. Nikon makes different versions of the 50mm f/1.4 lens. One version is the 50mm f/1.4 G AF-S and another is the 50mm f/1.4 D-AF. While only the second version bears the "D" tag, both lenses communicate distance. The first version bearing the "G" tag does not have an aperture control ring and would therefore be almost unusable on a body that does not have aperture control.
Summary
Don't worry about the "D" in newer Nikon lenses. It's there whether it's tagged or not. If you're shooting with a newer Nikon DSLR body, you have no practical use for an aperture ring so go ahead and buy the "G" type lens; it's also a "D."
Tags: aperture control, lenses have, Nikon lenses, aperture control ring, body camera, communicate distance, control ring