Non-polarised sunglass lenses, such as dark tint lenses just reduce the amount of light that gets through to the eye.  A polarising lens does the same but with the added feature of eliminating harsh glare. This especially noticeable around water, but also really useful driving in the low sun of winter and the brighter sun of summer. These are available as prescription as well as non-prescription. They also come as varifocals and bifocals. Have a look at this excellent version that Essilor make by clicking on the video.