The top answer covers the decoding of the letters very well. I thought I might add a few comments as to what some of the features actually mean in terms of consequences of the features (it took me a while to work out what some of them meant).
Lenses only for reduced frame DSLRs
Most low to mid range DSLRs have a sensor that is smaller than a 35mm film frame - sometimes called "reduced frame" or "cropped sensor". So using a "full frame" lens will mean lots of extra light around the sensor that isn't used. You can makes lenses smaller and lighter by reducing the projected image size to fit the sensor size. However using these lenses on a full frame camera would result in the corners of the image being dark - and mostly these lenses won't fit on a full frame camera.
The "less than full frame" codes are:
- Canon: EF-S (EF for full frame)
- Nikon: DX (FX for full frame)
- Pentax: DA (FA or D FA for full frame)
- Sigma: DC (DG for full frame)
- Sony/Minolta: DT
- Tamron: Di II (Di for full frame)
Image Stabilisation/Vibration Reduction
Image Stabilisation is also called Optical Stabilisation, Optical Image Stabilisation, Optical Steady Shot, Vibration Compensation and Vibration Reduction. Does what it says on the tin basically. (Although note that some DSLR bodies have a form of vibration reduction in the body and so don't need it in the lens).
- Canon: IS
- Nikon: VR
- Panasonic: OIS
- Sigma: OS
- Sony/Minolta: OSS
- Tamron: VC
Fast and Quiet Focussing Motors
The focussing motors in some lower end lenses can be quite noisy. The higher end lenses are able to focus more quickly (the movements can be more accurately controlled) and are quieter and use less battery. The acronym for it usually includes "Sonic":
- Canon: USM Ultrasonic Motor
- Nikon: SWM Silent Wave Motor
- Olympus/Zuiko: SWD Supersonic Wave Drive
- Pentax: SDM Supersonic Drive Motor
- Sigma: HSM Hyper-Sonic Motor
- Sony/Minolta: SSM Super-Sonic Motor
- Tamron: USD Ultrasonic Silent Drive
Lens Features
There are a variety of lens features to reduce chromatic abberations (where different colours don't exactly converge) and other imperfections in lens performance. In particular
Lens Coatings
There are a variety of lens coatings used to reduce internal reflections and other possible problems. Internal reflections can end up producing ghost images or adding to lens flare. Not all lens manufacturers specify the lens coatings they use.
- Nikon: NIC Nikon Integrated Coating, SIC Super Integrated Coating
- Pentax: SMC Super Multi Coating
- Sony/Minolta: T High-performance Coating
- Tokina: MC Multi-Coating
- Yashica: DSB Single-Coating, ML (later MC) Multi-Layer (later Multi-coating)
Macro
Macro lenses can focus very close to the end of the lens, providing (at least) a 1:1 ratio between the size of the object and the size of the image on the sensor. In plain english, you can take very close up shots of flowers, insects and so on. They are just called Macro (or occasionally Micro), making life easy for once.
Focusing
This includes Internal/Inner Focusing (IF) and (Internal) Rear Focusing (RF or IRF). Both of these reduce the number of individual lenses moving inside the lens. They also mean that the front of the lens will not move in or out, or rotate, during focusing. The lack of rotation can be important if, say, you have a circular polarizing filter, or a graded ND filter fitted to the lens. And the front not moving in or out can be important if the lens is very close to the subject.
High End Lenses
Some manufacturers have a code to indicate their high end lenses:
- Canon: L Luxury
- Pentax: * and Limited
- Sigma: EX Professional EXternal lens body finishing
- Sony: G Gold Series
- Tamron: SP Super Performance
Miscellaneous
Other codes might indicate the mount type (which will indicate whether it will fit your body), whether it will work with a Teleconverter or whether the lens needs the camera body to provide the motor for auto-focussing.
Note that I'm not an expert at this and am happy to integrate clarifications left in comments.
