Why do you need sunglasses?
Because I work for Oakley and this is all just a conspiracy to make me rich. Muhahahha. No, really!
Ignoring the everpresent fact that sunglasses can make you look infinitely cooler, to really understand the need for sunglasses we need to look at what they affect: the sun's rays.
The sun's rays arrive at three different energy levels: we call them infrared, visible, and ultraviolet.
We perceive infrared rays as heat; think of those big reddish lights you see over chickens at ShopRite; those're infrared.
Visible light is...well, visible. ROYGBIV wants to talk to you.
Ultraviolet (UV), however, is the tricky one. There are actually three levels of UV: UVA, UVB and UVC. The atmosphere takes care of UVC, filtering it out before it reaches us. UVA is also known as black light; it's the stuff that causes tanning. UVB, then, is where we'll focus. UVB is the current suspect in sun exposure ills: premature skin aging, wrinkles, and cancer, although considering that 99% of UV radiation at sea level is UVA, some people think UVA plays a not insignificant role in our solar problems.
The Problem
What UV radiation does is literally kill off your body's cells. Considering that these are your eyes we're talking about, you do NOT want the cells there dying. UV light has been linked to cataracts and macular degeneration, so one big thing we want sunglasses to do is to protect against UV radiation. You always want to get sunglasses that filter 100% of both UVA and UVB radiation.
We also want to shield against more mundane light-related problems. Intense glare, excessive light, and some frequencies of light we could live without; we'll let the sunglasses deal with these.
The Technological Solutions
The most important thing about a pair of sunglasses are the quality of the lenses. If you have a pair of crappy lenses that distort everything, your eyes are going to be killing you at the end of the day, just by looking through them.
Once you have a nice foundation to build upon - non-distorting lenses - you can begin to attack the light filtering. A few different technologies are currently used to achieve this: Tinting, Polarization, Photochromatic lenses, and mirroring.
Tinting
Tinting refers to the color a lens has. A lens can either have color all the way through or be clear, with a coat of color just on the surface. Differently tinted lenses will absorb different wavelengths of light.
Polarization
When you shine light through a light bulb, it is not polarized; the light waves are radiating outwards in all directions. Polarization is a way of filtering light so that all light passing through the filter radiates in only one plane. The physical polarizing filter is usually made of molecules that naturally align in parallel to one another. When applied to the lens, the molecules block any light that does not match their orientation. Polarizing filters are arranged so that all light is vertically polarized.
Photocromaticism
As mentioned above, photochromic lenses are ones which lighten or darken in response to UV light. Millions of molecules of a substance such as silver chloride or silver halide are embedded in the lens. While transparent in artificial light, the molecules change and absorb visible light when exposed to ultraviolet radiation. As mentioned above, photochromic lenses are all but useless when driving, as your car's windows filter out UV radiation for you. Photochromic lenses were developed by Corning in the late 1960s; you may remember the Transitions TV advertisements in the 1990s.
Mirroring
Mirroring makes use of the fact that you can see more out of small holes the closer you are to them, and you can see less through small holes the farther you are from them. Look through a keyhole or a piece of thin cloth and you'll be able to see this effect for yourself. Mirroring is a process in which a very sparse reflective coating is applied to the lenses - so sparse that this is sometimes called half-silvered, because only half of the molecules needed to make a mirror are present. This half-silvered surface will reflect half of the light that hits it and let the other half pass.
Sometimes the mirroring is applied in a gradient that changes density from top to bottom. If, for example, the top is more heavily mirrored than the bottom, you will not be blinded by the sun reflecting off of cars ahead of you while driving, and yet you will still be able to see the dashboard perfectly well.
The key problem with reflective sunglasses is that they are very scratch-prone, and sunglass makers have not yet devised a way to apply a scratch resistant layer over the silvered one.
Physical structure
Finally we have frame and lens design. While a conventional teardrop shaped lens will be sufficient for most people, in some high-reflection environment (such as skiing at Snowbird!), you'll need more protection. Here you'll want to buy wraparound frames that provide pieces of leather to protect light from getting in the sides of your face between your lenses and eyes.
Conclusion
All of these elements together can make for a perfect pair of sunglasses that could potentially save you from blindness! But whatever you do, don't forget your sunscreen. Or your towel.
Oh, yes: my source of information for this node was Marshall Brain's book How Stuff Works. | 
