This is actually two filters, a UV and a circular polarizing one. You can use them separately or stacked one on top of the other. These are well made filters, the ring is very small so they won't cause any vignetting even when stacking both filters on fairly wide angle lenses. These threaded onto my lens smoothly and stayed on securely. They only reduce the a little, and once they are screwed on there is an outer ring that lets you rotate the polarizer to get the best effect.

The UV filter helps reduce atmospheric haze and makes the air look clearer. A lot of people also leave a UV filter on to protect the front element of their lens from dust and splashes. The Circular Polarizer will help reduce reflections from things like water, glasses and windows. Because it reduces reflections, you can see down into water making it look clearer and cleaner. It also helps deliver a clearer sky image when taking photos outside.

This is the "K" series filter, they also have a "D" series. I'm not sure the exact difference between the two as K&F's own website doesn't really explain it. As near as I can tell the "K" series ones (this one) are slightly cheaper with less layers layers of coating, the K series doesn't seem to have a water/oil repellant coating.

In any case, well made filters that do the job.