Sunlight contains light that we use to see with, but also invisible, ultraviolet light that is extremely harmful to the eyes. Ultraviolet light, usually called UV light, is very high energy and can damage several parts of the eye: the cornea, the inner lens of the eye (causing cataracts), and the back of the eye (causing macular degeneration). It is the cumulative effect of the UV light over time that damages the eye, just as long-term exposure to UV light damages the skin. It is very wise to protect your eyes from the UV light by wearing sunglasses.
Indoor light and light from cellphones or computers does not contain harmful levels of UV light.
All sunglasses sold legally in the United States protect against UV light. The sunglasses do not have to be dark – because the UV light is invisible, the sunglasses can be completely clear and still remove the UV light. The key is to use them when you are outdoors in the sunlight. When you are in your car, the windshield removes most of the UV light, so it is not necessary to wear the sunglasses in the car (unless you have your top down in sunny southern California!).
Polaroid lenses decrease the glare produced off of flat, reflective surfaces. They do not decrease any other type of glare. Reflection of light off of flat surfaces such as roads, glass, and particularly water produce glare that is decreased by use of polaroid lenses.
Is the color of the sunglass lenses important? Not really, though each person will have a personal preference. Grey lenses give the most accurate color. Yellow lenses are good in lower light levels, such as overcast days.
Are expensive sunglasses better? As far as the lenses go, there is really very little difference in quality between expensive and inexpensive sunglasses. The cost is almost all related to fashion, not vision or protection of the eyes.
After your vision is improved by Laser Vision Correction, it is great to be able to use fun, non-prescription sunglasses!