AF = Auto-Focus

These lenses have a unique mechanical coupling which attaches to your Nikon DSLR, which then moves the lens. There are no motors in AF lenses, so your DSLR will determine the speed at which it focuses. On a side note, the Nikon D40 and D40x do not have internal motors themselves to use the coupling device on non-motorized lenses. Your DSLR must have an internal motor itself in order to use the auto-focusing feature of lenses which do not have their own motors to drive the focusing system. So, for example, only AF-I and AF-S lenses will work with a D40 and D40x.

AF-I

These lenses have a small "micro" motor inside the lens, and were unique to Nikon super-telephoto lenses in the early 1990's.

AF-S = Auto-Focus + Silent Wave Technology (Ultrasonic Technology)

This is a step-up from your ordinary AF lens. Ultrasonic motors are used in these kinds of lenses, making the movements of the lens element groups, virtually silent. Ultrasonic waves are used to move the lens elements back and forth. AF-S lenses are also famous for lighting-fast focusing. Again, this is becaue of the ultrasonic technology (Canon also uses ultrasonic motors).

D = Distance

The "D" on Nikkor lenses, is in reference to distance. An internal encoder is connected to the lens focusing ring, and transmits the shooting distance information to the camera automatically. 10-segment Matrix Metering is an example of what a "D" lens can do when attached to your D-series DSLR. A Lens without a "D" means you will not be able to meter with your D-series DSLR.

DX = 24mm x 16mm

Nikon's latest and newest type of lens, which is specifically engineered for the D-Series DSLR, by making a smaller image circle lens, enabling wider angle capabilities at a lower price than what a similar lens would sell for. Nikon's 24mm x 16mm imager size in their D-series cameras, are referred to as a "DX" format. This is why Nikon has released lenses with the DX name.

Currently, we have only seen the AF-S 12-24 DX lens, but other versions are on their way, Nikon says. The only problem associated with the DX lens, is the price factor. The AF-S 12-24DX was originally selling for $1250, and is now around $1000. However, the price still needs to come down to around $800 or less. The main reason for this is because Nikon was very public about their "cheaper" DX lens.

ED = Extra-low Dispersion

The farther you zoom, the more CA (Chromatic Aberration) is apparent. ED Lenses are made to reduce CA and are used in the more expensive zooms. Chromatic Aberration happens due to basic physics of light. A lens will not focus different colors in the exact same place, because each color has its own wavelength (Blue focuses nearer than Green or Red wavelengths). Thus, you need different lens elements to focus at specific color wavelengths. This is why some lenses are so expensive, and have multiple lens elements in them. An example of CA in a photo would be taking a photo pointed at a subject that has several edges, such as leaves on a tree, or a building with many different architectual edges poking out. CA would be prevalent if you notice purple-ish "blooms" around the edges of the subject.

If I have a photo that suffers from CA, I use Photoshop. Any photo editor would do the job actually, as long as you have individual control of the colors (Hue and Saturation for example). So, if you have an understanding of Photoshop, you can most likely take care of CA without having to get an ED lens. However, the extra money you spend on them is worth it.

G = No Aperture Ring on Lens

G Lenses have been around for a little while, but Nikon's earlier "G" lenses were not so great. The only reason I say this is because I have heard this first-hand from several Nikon users of the 70-300G and the 28-80G lens. The newer "G" lenses, such as the AF-S 24-85G, and the 70-300 "D" version, are not so bad.

IF = Internal Focusing

Internal Focusing on certain kinds of lenses, particularly telephoto lenses, allows for a much more balanced lens when shooting. The lens focuses internally, needing no more room to focus than the length of the lens itself, so you don't have to worry about buying a 12" lens and ending up with a 24" lens after zooming completely in on a subject, and having to counter-balance the effect. A "Push/Pull" lens does exactly this: the lens moves in and out of its main barrel in order to focus. Camera Shake is very hard to control in these situations, and you must have a pretty good grip on the camera and lens itself to avoid this when you are shooting in not-so-perfect lighting conditions (which is basically most of the time).

VR = Vibration Reduction

VR (Vibration Reduction) lenses are beginning to appear much more often from Nikon. What this technology does, is allow for lower shutter speed photos, without the blur that is normally associated with it. The 80-400VR was the first to use this technology, and the latest lenses are the 70-200G VR and the 24-120G VR.