Your 6D can work with any Canon EOS "EF" series lens (but not the "EF-S" lenses or "EF-M" lenses - those are made exclusively for crop-frame bodies and mirrorless bodies). You can also use any of the speciality lenses such as the TS-E (tilt-shift) and MP-E (macro photo).
The "EF-S" lenses are "Short" back-focus lenses. These are made specifically for Canon APS-C crop-frame bodies such as the Rebel line, the 60D and 70D, as well as the 7D. Since these bodies have a physically smaller image sensor, they don't require a lens that can project a large image circle onto the sensor. That means they can use smaller lens elements while still maintaining quality.
The "EF-M" lenses are for the new Canon "mirrorless" bodies -- of which the EOS-M is currently the only camera in that category. They have a much shorter back-focus distance because the camera does not require space for a reflex mirror between the sensor and the lens. (the "back focus" distance is the distance from the rear-most lens element to the sensor plane.)
Canon has auto-focus lenses which don't have any special tag for the focus motor at all (they don't specifcy either "USM" or "STM"). These are the simplest auto-focus lenses and are generally found on the entry level lenses. The focus motors are not particularly fast nor quiet. If you were shooting a sports game with rapidly changing focus distances, you may find that the focus motors cannot keep up with the action. The focus system engages with a positive gear... you should _not_ attempt to manually focus one of these lenses if the auto-focus mode is switched on. Also... as these tend to be entry-level lenses, the lenses usually will not have internal focusing or internal zoom. That means that as you focus, the front-most part of the lens rotates as it focuses. That might not sound like a big deal but there are times when you might have an effects filter or other filter on the lens where the orientation of the filter is important... and when you focus the lens, the orientation of the filter changes. The most common case would be when using a "polarizing" filter. As the lens focuses, the lens rotates (and the filter with it) which de-tunes the filter.
"USM" means the lens has Canon's "UltraSonic Motors". These are the motors that drive the focus system on the lens. The UltraSonic motors are the fastest focusing motors. They're fairly quiet but not AS quiet as the new STM motors. The lenses that have USM motors also have a focusing clutch... this means you get full-time manual focus even while in auto-focus mode. You can grab the focus ring and twist it at any time. If the camera isn't driving the focus, then the focus ring will move the focus on the lens. But if the camera is driving the focus then the focus ring is allowed to slip so that the lens will not be damaged if you focus in one direction while the camera tries to focus in the opposite direction. Most USM lenses have "internal focus" -- meaning the lens does not actually rotate as you focus.
"STM" means the lens has Canon's new "STepper Motor" technology. These are electronic focus motors which are _extremely quiet. They are not as fast as the USM motors but they are much faster than the basic motors. They tend to be very smooth. They are deliberately designed to be quiet so that the noise of the focus motor is not audible when recording video.
The "II" (and sometimes "III") is just a generation number. From time to time, Canon will revise a lens even though it's base specs are the same. For example, I own an EF 24-70mm f/2.8L USM and it's the original Canon 24-70mm f/2.8L USM lens. Canon has subsequently introduced a new version of the lens with several key differences, but the lens is called the "EF 24-70mm f/2.8L II USM". The "II" simply means the lens has been revised and this is the 2nd generation lens. Just occasionally there's even a "III". I have never encountered a "IV".
One last thing... some of Canon's lenses have the letter "L" (often in red) listed after the focal ratio. For example: EF 24-70mm f/2.8L USM. The "L" indicates that it's one of Canon's high-end "Luxury" lenses. These lenses tend to have the very best features... generally a very solid and durable build quality, the highest image quality, and numerous other quality features. They are often weather-sealed lenses. You can spot them from a distance because they also paint a red stripe around the end of the lens.
Tim Campbell
5D III, 5D IV, 60Da
