Let's start out with the technical definition: UV stands for Ultra-Violet, and UV filters are designed to keep Ultra-Violet rays from entering the lens. Here's a simpler one: a UV filter is a clear piece of glass. The filter won't alter your photographs: it won't change the colors or make things look blurry.
So what's the point of a clear piece of glass?
The clear UV filter screws onto the front of your lens. Imagine that you're on a hike and grind the front of your lens against a sharp rock. Without a UV filter in place, you're going to have to replace the lens. This can cost anywhere from $300 to $3000 (depending upon the lens).
With a UV filter in place, the only thing you've damaged - and need to replace - is the filter. Approximate cost: $30.
What's the Catch?
Since the logic behind a UV filter makes so much sense, why doesn't everyone use them all the time? Most people do, but there are some professional photographers who argue that a UV filter affects the quality of your photos regardless of the filter's clarity. This argument also makes sense: for the clearest possible images, don't put something on the front of your lens and leave it there all the time. In the end, the answer has a lot to do with how you use your camera:
- If you enjoy taking your SLR on your travels, hikes and other excursions, where there is a high chance of smacking the front of the lens on something, then always use a UV filter
- If you take photos in more controlled environments (indoors, studio, etc.) then you may opt to leave off the UV filter to ensure the highest possible image quality
