While you probably understand how important it is to wear sunglasses during summer, you should know it is also important during the winter. Believe it or not, the snow can reflect quite a bit of light. Even if there isn't any snow, the sun can still be quite bright in cloudy weather.
There is great disparity of quality when it comes to sunglasses, and some do not protect at all. The ones you wear should completely block all UVA and UVB sun rays. While you might look at sunglasses with just your fashion sense, you must understand that cheap sunglasses are going to negatively affect your vision.