APO stands for apochromat. APO lenses have lens elements that reduce chromatic distortion like thin colored lines along edges and transitions in the photo, so you'll get a higher image quality.

Lenses in the ART series offer the highest image quality and they contain high-end lens elements. These lenses have large apertures like f/1.4. This way, you can play around with depth of field and you can take photos at dark locations.

ASP means aspherical. This means the lens elements aren't cut like a sphere, so they can correct possible lens deviations.

Contemporary is a Sigma lens series. These lenses have the latest techniques and the design is very compact. The design is compact, but the lens doesn't lose quality. In this series, you'll find standard lenses and telephoto lenses.

Converter. Lenses with this indication are suitable for APO Teleconverter EX, which extends the focal point and range of the length.

The abbreviation DC stands for Digital Calculated. These lenses are designed for cameras with an APS-C sensor.

Digital Grade lenses are designed for cameras with a full-frame sensor, but you can also use them in combination with APS-C cameras.

DN stands for Digital Neo. These lenses are developed for Sony SEL and NEX, Panasonic Lumix G, and Olympus PEN mirrorless cameras. Lenses with a DN abbreviation aren't compatible with full frame and APS-C SLR cameras.

Extra Quality means that the lens is from the most high-end Sigma series.

Lenses with an HDM have a Hyper Sonic Motor. This is an autofocus motor that provides a very fast, silent, and very accurate autofocus. The motor uses ultrasonic sound waves to do so.

With lenses with Inner Focus, only the inner lens group moves during focusing, so the physical length of the lens doesn't change.

OS means that the lens has an Optical Stabilizer. This system compensates the vibrations that can cause motion blur.

RF means Rear Focus. With these lenses, the rear lens group moves to focus. This provides a faster autofocus.

Sigma has a lens series for Sports. These lenses have high-end lens elements and a fast HSM autofocus motor. This way, you can also take photos of fast-moving objects like athletes and wild animals.

UC means Ultra Compact. These lenses are very compact, like the name suggests. That's useful for travel photographers.