ND filters fit on the front of the lens and limit the number of light that enters the camera. I bought this one to use with my Nikon 18-55mm lens and really enjoyed playing with it. The main reason I got it was to make water look better. If you have every seen a picture where there is a river or body of water that looks silky smooth, they probably did that with and ND filter. I don't use it much more, only because I don't use my 18-55mm lens much since I purchased a 18-200mm lens. The problem with these filters is they only fit on a lens of the same size (thats where the 52mm comes from), so I will probably sell this filter when I sell my 18-55mm lens and buy a new one for either my 18-200m lens or maybe the 12-24mm lens that I use for landscapes.
To get a better idea what a photo with an ND filter looks like check out the photos I have posted where I used this ND filter.