What do all those cryptic number and letter codes in a lens name mean?
When looking at a lens name, there are a lot acronyms describing its features (often specific to the manufacturer).
Nikon AF-S DX 16-85mm VR f/3.5-5.6G IF-ED
Nikon AF-I 600mm f/4D IF-ED
Nikon AF-S VR Micro-NIKKOR 105mm f/2.8G IF-ED
Canon EF 85mm f1.2L USM Mark II
Canon 70-300mm f/4.5-f/5.6 DO IS
Sigma 150mm F2.8 EX APO DG HSM Macro
Sigma 70-200mm F2.8 EX DG OS HSM
Sigma 50-150mm F2.8 EX DC APO HSM II
How do I decipher these lens codes from different manufacturers?
I have made this thread a CW thread. The answers here are extensive and very useful, but could still use enhancement and updates. I encourage the community to add to the answers below if they notice anything is missing. Please conform to the format of each existing post, and do not heavily modify the existing format of each one. Additions and corrections to the existing contents are encouraged.
Note: Because of the community-wiki nature of this question, this accepted answer became really long, and difficult to edit and keep current as lens designations evolved. The historical answer has been broken up into individual answers per lens brand, with links to each of the lens brand answers below.
Descriptions by Lens Make
All major camera manufacturers offer their own line of lenses. Such lenses tend to follow the most stringent quality guidelines, and often come with a price premium.
Most off-brand lens manufacturers make lenses that fit many types of bodies, including Canon, Nikon, etc.
Descriptions by Lens Features
If you know what feature you're looking for in a lens (cropped-frame designation ultrasonic motor, low-dispersion elements, image stabilization, etc.), and want to know what each of the brands call that feature, the following answers are organized by lens feature.
@Lazer: That would generally be specific to each lens. A lot of the time, differences between a I and a II or a II and a III are subtle, but important. Other times they are significant. I recommend reading http://www.the-digital-picture.com/Reviews/ for details on lenses.
In the Nikon section you forgot as well the PC and PC-E features for Perspective Control and E for Electornic diaphragm (to be confirmed)
@Lazer In the Canon world, Mark II (III, etc.) versions of a lens must have the same aperture, IS, and focus motor specs as the original. If any one of those changes, then the new lens is not called Mark II (e.g. 50mm f/1.8 micro-motor Mk II vs. 50mm f/1.8 STM). Therefore, usually a Mark II lens signifies an upgrade in sharpness or weather sealing. However, the exact changes vary from lens to lens, and some Mark II's have no real benefits (such as the 24-105mm f/4 L IS).
There's no such thing as a "Mark I" camera or lens in Canon nomenclature. There's the original "*XYZ f/1.2.3*" lens and then a later "*XYZ f/1.2.3 II*" lens if the lens has the same focal length(s), maximum aperture(s), and AF type. There's the original EOS *xxxxD* camera body and then a subsequent *xxxxD Mark II* camera body. The word "Mark" in front of the roman numeral II, III, or IV is only used with camera bodies. It is not used at all in the names of Canon lenses.