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UV filter

Ultra Violet is a light blue colored section in the light spectrum that can hardly be seen by the human eye. It is on the edge of the visible light. But a camera sensor does sense the UV light. Actually, it is highly receptive for this wave length. Part of the UV light is absorbed by the glass from the lens itself, but in environments were there's a larger share of UV in the ambient light, blueish images can occur and they tend to look unsharp too. Where can you find these environments? Up in the mountains, close to large arrays of water or on large open fields. The problem is that with the human eye this blue haze is hardly visible. So it's better to protect yourself against it with a UV filter.

The negative effect of UV light is not so much the colorization of the image, but the overal reflection and disturbance on the light measurement. A surplus of UV light entering you lens and your camera will reduce the overal quality of your images, by decreasing the sharpness and detail and effecting the exposure value.

Skylight

A skylight filter is a combined UV and color filter. Not only diminishes it the UV light rays passing through, it also has a moderate pink tone. This pink colorization will compensate the blue of the UV light and so a skylight filter has a stronger effect.
But skylight filters are obsolete nowadays, because digital cameras are equipped with white balancing instruments so the pink colorization will be automatically become undone by the camera's color correction. The only effect of a skylight filter is that it reduces the light transmission a bit. That's no use at all. Let's abandon skylight filters.

Protection

When photographing in environments with a high share of UV light, a UV filter does its job. But what if you are in normal conditions? In that case a UV filter should do nothing at all. Which is, quite frankly, not alway the case, since a lot of brand-less or cheap filters do effect other color regions of the light spectrum too. But that aside.
Still there's a reason that many photographers, amateurs and professionals alike, always keep a UV filter mounted on all their lenses: as a protection. Sure, it works, but for that purpose you better use a dedicated protection filter.