Sunglasses are basically a pair of semi-transparent pieces of glass, held in front of your eyes in order to protect them from bright light. Or at least that's what any conventional encyclopaedic institution would tell you. An optician would claim that the sunglasses' ability to shield your eyes from the harmful ultraviolet rays of the sun is their single most important aspect; any fashion-designer, fighter-pilot or if you are a Terminator1 would argue fiercely that looks are much more important, and, of course, the latter would be right.
The Science Bit
The sun sends out a lot of radiation. There's the visible light which covers a certain area of the spectrum, this ranges from red to violet. Below the visible light in the spectrum there's the heat-radiation which we sometimes call infra-red radiation, microwaves and radio waves. At the other end of normal light, there's ultraviolet radiation (colours we can't see), and further up there's X-rays, gamma waves and so forth. Ultraviolet radiation gives us a great tan, but it can damage your eyes as well as cause skin cancer. Thus it's a good idea to use sun screen and to wear sunglasses as a preventive measure.
The Stylish Part
The most important function of sunglasses is of course the style factor. The uses of a pair of sunglasses are many, too many to mention here, but wearing a pair of sunglasses can add a certain flair to any normally-clad individual.
A person whose eyes you can't see seems to be more interesting and aloof than one without sunglasses. Beware, however, since a person with sunglasses may also be very fashion-sensitive, depending on whether or not sunglasses are in fashion.
You'll appear all the more interesting for wearing a pair of sunglasses, which can't possibly be a bad thing. This depends quite heavily on which kind of sunglasses you choose. There are many kinds, for many uses:
Pilot sunglasses - are worn by policemen, pilots, Central American dictators, truck drivers and others to whom looking very macho is important. Do they use them in order to seem distant and unapproachable, or in order to hide their scared eyes? Nobody dares go up to one of these individuals and ask, as they usually look quite intimidating.
Popstar sunglasses - are utilized by superstars when they are being interviewed on television. They also give the impression that a popstar is a superstar, and that their ridiculous responses to easy questions are cool. Or it could make you think that they're wearing them because they've had a few nights of very heavy partying.
'On the beach' sunglasses - are used on beaches and other places one goes to look at persons of the opposite sex (or the same, depending on your taste). When you're wearing them no-one can see what you're really looking at. Women can use them to glance seductively over, which is a recurring fantasy for most men.
Sports sunglasses - sports icons usually have very fancy and expensive glasses. This isn't because you actually need to wear a pair of glasses in order to run 100 metres or jump over a stick. It's because the sponsors want to sell them. They're very fancy and look expensive, and that's sometimes the only reason some people feel the need to buy a pair anyway.
'Hippie' sunglasses - are often completely round and much too small, or star-shaped and bright green, are meant to symbolize that you don't believe in the materialistic society, and, that you think we should all hold hands and have sex and smoke something illegal. As worn by John Lennon of The Beatles.
Tough-guy sunglasses - are of the kind worn by Arnold Schwarzenegger in Terminator, Will Smith in Men in Black and Keanu Reeves in The Matrix. Whether this is because they're supposed to act as some sort of protection against flying shrapnel or just because sunglasses are cool is unknown. But, hey, sunglasses are cool.
Sunglasses are used to heighten your 'cool' or hide your face. However don't, no matter how much you need a little extra 'cool', wear sunglasses at night. It just looks silly, and desperate. Don't wear them to work either, they're definitely a leisure item. Wearing them at the office will just make people think you're either crying or trying to hide a damaged face. Wearing them also makes it harder to see what's on computer monitors. Good point.
Sir Elton John has admitted that he owns a quarter of a million pairs of spectacles, and most of them have coloured lenses.