Cycling Sunglasses 101
Cycling glasses can be found on many cyclists, for good reason. They serve as much in function as they do for fashion. We'll outline some of the functional aspects here; fashion is purely up to you.
Protection for your eyes
Cycling eyewear's primary function is to protect the eyes both from impact with road/trail debris and from the sun. They are generally impact resistant and made of quite sturdy plastic in order to defend your eyes against things like pebbles, sand, sticks, or anything else your tire or the tire in front of you kicks up. Cycling glasses also help keep water out of your eyes in the rain and bugs out when those little buggers are everywhere.
For protection from the sun, there are different shades of lenses that you can choose from to tailor the level of light being emitted to your eye. Some lenses are photochromic so that they darken as more UV light hits the lens, giving them a range of light filtration within the one lens.
These are very popular for cycling sunglasses as they allow the ability to interchange lenses so you can have one frame but select the different lenses for different conditions. Having glasses with interchangeable lenses is the best option for those that ride in many different conditions and don't want to buy several sets of glasses. They usually don't cost much more than single-lens types either.
As an added benefit, if you scratch up one set of lenses, you will have others to place into the same frame to continue using the glasses (you'll just have fewer options). You can also purchase replacement lenses for many glasses with interchangeable lenses.
A polarized lens works by allowing only a certain orientation of light wave through the lens. What this does is reduces glare and overall light intensity. It also allows a clearer view of your surroundings since there is less "noise" light. They help the most when there are reflective surfaces in the vicinity, like water.
Vented lenses are offered on bike glasses because they help reduce fogging. The vents allow more air to pass over the lens, which decreases the chance of condensation. There are different sizes and style of vents, but there's no clear indication that one is better than the other. Just know that having them is better than not if you have troubles with lenses fogging up.
Different frame designs offer different benefits, but first you want to make sure that the glasses fit. Comfort is important if you're going to be wearing these for long periods of time. Some frames contact the helmet oddly and may contribute to discomfort. If possible, try them on with the helmet, or look for helmet-friendly features.
Different frames are also designed around different face widths. Some clearly stating what size they're intended for,but many do not. There's not a good standard for face width at this time, so trying on glasses is best for deciding if the fit is good.
Open-frame styles offer the best visibility and often are the best at preventing fogged lenses. They can be flimsy feeling and not grab the head well, though.
Closed-frame glasses offer the best rigidity, but the frames can give the sensation of improvised viewing at wide viewing angles like those in your peripheral vision or when looking back over your shoulder to check traffic.
More common are glasses that have the frame running just along the top, leaving the bottom edge open. This design gives you the best of both worlds as it lets liquid run off the bottom but still has reasonable rigidity.
Some other features to look for in the frames are grippers -- usually a silicone or rubber piece -- on the arms that help hold the glasses in place when you're moving your head around quickly. It could also help keep them from bouncing off if riding on rough terrain. There are different styles of nose pieces, as well, with some having adjustability and some that are one size. Make sure you get something that has enough surface area in the pad to set on your nose bridge without cutting into the skin too much but also provides enough grip to help keep the glasses in place.
Getting the right cycling glasses comes down to figuring out your uses for them, trying some on and seeing what works. There will be some trial and error, but having eyewear to protect your eyes is a very good idea. Think of them more as safety glasses than sunglasses, since they do more to protect the eyes than simply help dim the lights.