AF

Autofocus.  Lenses with this prefix have the older "screw drive" type autofocus, where an in-body motor connects with a screw on the lens in order to drive the lens elements.  This type of lens will not focus on newer bodies that do not have an AF motor, imcluding (but not limited to) the D40 and D60.

AF-S

"Silent Wave" Autofocus.  Almost all newer lenses have this type of in-lens AF mechanism, which consists of a ring-type "ultrasonic" motor that drives the lens elements.  Usually somewhat quicker and quieter than the "AF" system.

Sigma: HSM ("Hypersonic Motor")

DX

Lenses with this identifier only project an image circle that is big enough to cover an APS-C (ie. not FX or "full frame").  There is nothing to stop these lenses being mounted on an FX camera, but at most focal lengths the image will have to be cropped to prevent black borders or significant corner shading.  There is no equivalent "FX" identifier; all lenses without "DX" work on both DX and FX cameras.

Sigma: DC

Nikkor

This is Nikon's brand name for lenses.  Originally used only for Nikon's (actually Nippon Kogaku K.K., Nikon's original corporate name) highest quality optics, but now used for all Nikon lenses.  This also includes lenses not compatible with Nikon DSLR's, such as enlarger and rangefinder lenses.

D

This means that the lens communicates distance information to the camera body, in order to aid functionality such as metering and AF.

G

As "D", but also signifies that the lens does not have an aperture ring.  The aperture can only be set via the control dials on the camera body itself.

VR

Vibration Reduction.  Lenses with VR (now at version 2, or "VRII" on newer models) attempt to automatically compensate for camera shake by using feedback from an in-lens gyroscope to slightly shift lens elements.  This allows the use of longer shutter speeds whilst still hand-holding the camera, and is most useful on telephoto lenses.

Sigma: OS ("Optical Stabilization")

IF

Internal Focusing.  These lenses do not turn the front element when being focused, which is an advantage when using some items attached to the front element (such as a polarizing filter).

ED

Extra-low Dispersion glass is used for some lens elements.  This type of glass helps prevent chromatic aberration (and "purple fringing") as it concentrates all wavelengths of light more equally than other kinds.

N

Nano coating.  A new type of lens coating developed by Nikon, which is supposed to result in less flare and ghosting.

PC

Perspective Control.  Mainly intended for architectural and macro use, the front element can be shifted in relation to the other elements in order to correct for perspective or change the plane of focus.

DC

Defocus Control.  Only present on a small number of specialist lenses intended for portraiture use, this capability allows the photographer to alter the appearance of the out of focus areas of the image.

Fisheye

These lenses produce a very wide angle of view, with considerable "barrel" distortion.  Horizontal and vertical lines in the image appear curved, with the effect increasing the further from the center of the image they appear.