The top answer covers the decoding of the letters very well. Here are a few comments as to what some of the features actually mean in terms of consequences of the features.
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 EOS dSLRs: EF-S (EF for full frame)
- Canon EOS R mirrorless: RF-S (RF for full frame)
- Canon EOS M mirrorless: EF-M
- 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)
- Tokina: CF or [older lenses], DX (FX for full frame)
Lenses only for mirrorless cameras
Many brands made/make both dSLR and mirrorless cameras and lenses. Typically, dSLR lenses can be adapted to mirrorless, but mirrorless lenses cannot be used on dSLRs.
- Canon: EF-M (EOS M system); or RF and RF-S (EF/EF-S for dSLR)
- Nikon: Z
- Sony: SEL (SAL for dSLR)
- Olympus: M.Zuiko
- Panasonic: Lumix G or DG (D for dSLR)
- Sigma: DN
- Tamron: Di III
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. (Some camera bodies — notably, Olympus and Pentax — have a form of vibration reduction in the body and so don't have it in the lens).
- Canon: IS
- Fujifilm: OIS
- 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 or newer DC Direct Current
- Sigma: HSM Hyper-Sonic Motor
- Sony/Minolta: SSM Super-Sonic Motor
- Tamron: USD Ultrasonic Silent Drive
- Pentax: WR Weather Resistant or higher-level, AW All Weather (also found on ★ lenses).
- All Olympus PRO lenses are weather sealed.
- Nikon: Nikkor AW lenses (Nikon 1 system) are waterproof to 15 or 20m
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
aspherical lens elements have a more complex surface profile that allows for better image quality in exchange for increased cost.
low dispersion glass is more free of chromatic aberration.
apochromatic denotes a lens which is highly corrected for color, bringing three colors (usually red, green, and blue) into equal focus.
Canon: DO Diffractive Optics (Canon does not include information in a lens' name regarding any fluorite, aspherical, low dispersion, or apochromatic lens elements that may be included in the lens' optical formula.)
Nikon: ED Extra-low Dispersion Glass, ASP Aspherical Lens Element
Olympus/Zuiko: ED Extra-low dispersion glass
Pentax: ED Extra-low dispersion glass, AL Aspherical Lens Element
Sigma: ASP Aspherical lens element, APO Aphochromatic (low-dispersion) lens element
Sony/Minolta: AD Anomalous Dispersion, APO Apochromatic correction using AD elements, HS-APO High-Speed APO
Tamron: Aspherical or ASL aspherical lens element, AD Anomalous Dispersion, ADH AD + ASL hybrid lens element, HID High Index, High Dispersion Glass, LD Low Dispersion, LAH LD + ASL hybrid lens element, XLD Extra Low Dispersion, XR Extra Refractive Index Glass
Tokina: AS Aspherical lens element, F&R Advanced Aspherical lens element, HLD High-Refraction, Low Dispersion, SD Super Low Dispersion
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
- Fujifilm: EBC Electron Beam Coating, Nano GI Nanotechnology Gradient Index
- Pentax: SMC Super Multi Coating, SP Special Protect, HD High Definition
- Zeiss: T* (pronounced "T-Star") High-performance Coating
- Tokina: MC Multi-Coating
- Yashica: DSB Single-Coating, ML (later MC) Multi-Layer (later Multi-coating)
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.
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 graduated 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.
Aperture Control Ring
Now that most camera bodies can control the lens's aperture, some manufacturers have a special designation for whether a particular lens has an aperture control ring:
- Nikon: G lenses (having the letter "G" immediately after the maximum aperture designation) do not have an aperture control ring on the lens.
- Fujifilm: R lenses (having the letter "R" immediately after the maximum aperture designation) have an aperture control ring on the lens.
Some manufacturers have a code to indicate their high-end lenses:
- Canon: L Luxury
- Fujifilm: XF with red badge
- Pentax: ★ and Limited
- Sigma: EX Professional EXternal lens body finishing. "Global Vision" lenses are badged A (Art), S (Sport), or C (Contemporary). A and S lenses are considered premium.
- Sony/Minolta: G Gold Series, GM G-Master Series – a newer (Sony-only) higher-end series
- Tamron: SP Super Performance
- Olympus: a PRO label next to the aperture marking
Some manufacturers include an apodization filter in the lens to improve bokeh:
- Minolta/Sony: STF (lit., Smooth Transition Focus)
- Fujifilm: APD
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.