Light rays which are reflected become polarized. Polarizing filters are used to select which light rays enter your camera lens. They can remove unwanted reflections from non-metallic surfaces such as water or glass and also saturate colors providing better contrast. The effect can be seen through the viewfinder and changed by rotating the filter. The filter factor varies according to how the filter is rotated and its orientation to the sun. Filter factor is between 2.3 and 2.8 (approx. +1.3 stops).
Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems). This filter is a must for all outdoor sunny or snowy high glare situations. It will cut out the extra bright glare and allow you to get the true blue colors, not washed out. Most photographers view this filter as "sunglasses" for your camera lens.
Circular Polarizing Filters are used to increase color saturation, like darkening blue skies so that white clouds stand out. It eliminates reflections from non-metallic surfaces like glass and water. It helps reduce softness in scenery caused by atmospheric haze. This filter will not change the overall color balance of a photograph.
Polarizers are commonly used to control glare on water and to allow the camera to see below the surface. They are also used to reduce glare on car bumpers and to control reflections on plate-glass windows. Polarizers are so versatile that they can perform the opposite functions as well. Some cinematographers use polarizers to increase or enhance reflections, simply by changing the filter’s setting. "Circular" refers not to the shape, but a special design that does not interfere with metering of auto exposure or auto focus systems