Rather than considering the amount of protection that they afford, new sunglasses are often selected for their frames, the size of the lenses, or for a particular coating on the lenses. It is important to remember, however, that the most important job for sunglasses is to prevent damage to the eyes from the sun's harmful rays. Fortunately, with a little knowledge about Ultraviolet (UV) radiation and sunglasses technology, fashion does not have to take a backseat to safety. 

What does UV protection indicate?

The amount of UV protection is the single most important factor to consider when shopping for sunglasses. The three types of invisible high-energy UV rays are UVA, UVB, and UVC.  At present, UVC waves are being blocked by the earth's ozone layer and are not a factor in causing eye damage. UVA and UVB rays are the ones that are most damaging to the eyes. Fortunately, even inexpensive sunglasses can provide complete protection against these rays. Always select sunglasses with labeling that indicates that they block "99 to 100% of UVA and UVB rays". If this is not indicated on the label, pass them up.
What other features provide help protect the eyes? In general, larger lenses provide more protection than smaller lenses. In addition, "wraparound" styles of sunglasses help prevent sunlight from reaching the eye from the sides of the glasses.

Why are some sunglasses more expensive than others?

This has to do with a number of factors, such as the optical clarity of the lenses, the exclusivity of the frames, or the special coatings that are applied to the lenses. So long as they are both providing complete UV protection, however, a twenty-dollar pair of sunglasses can be just as protective as a two-hundred dollar pair.  Typically, however, the more expensive brands of sunglasses, include a higher quality lens capable of producing a clearer, sharper, image with less distortion.
Does being darker mean that the lens is more protective? How deeply tinted the lenses of sunglasses has no bearing on the amount of protection that they provide. Lightly tinted sunglasses that block 99 to 100% of UV rays are more protective than a darker pair with a lower UV rating.

What about the protection provided by Transitions® sunglasses?  

Transition lenses are also known as "photochromic lenses" since they darken on exposure to UV light. Their manufacturer indicates that they provide 100% UV protection whether they are clear or as dark as regular sunglasses. Not all manufacturers of photochromic lenses offer complete UV protection, so be sure to check the product label.

What is the advantage of polarized glasses?

Polarized glasses are best for preventing glare, particularly when near the water. That makes them particularly useful when fishing or boating. They can also prevent eyestrain from glare when driving. In most cases, polarized sunglasses also provide complete UV radiation protection, but this is worth confirming before purchasing.

Are certain lens colors better than others?

As long as the lenses are providing complete protection against UV radiation, the American Academy of Ophthalmology indicates that color is a matter of personal preference. Gray, green, or brown lenses are the most popular shades of lenses for sunglasses.

What about shaded lenses?

These are popular with many designer sunglasses manufacturers. They are also known as "gradient lenses", since they are permanently shaded from top to bottom or top and bottom toward the middle. This variation in tinting of the lens does not affect the UV protection and can be as protective as sunglasses with a uniform tint.
From this discussion, it becomes clear that eye protection from the sun and stylishness can co-exist. The most important issue is to make sure that the sunglasses you choose provide complete protection against UV rays. Be aware that even though contact lenses can provide some UV protection, sunglasses should be worn outside since contacts do not cover the entire eye. Also, considering the amount of time that children spend in the sun, they should wear UV-protective sunglasses also.

Sources for article:
How to choose the best sunglasses from the American Academy of Ophthalmology
Guidance Document For Nonprescription Sunglasses from the U.S. Food and Drug Administration