From the Manufacturer 
Light rays which are reflected by any surface become polarized and polarizing filters are used to select which light rays enter your camera lens. Circular polarizing filters have the same effect, but it is important that you choose the correct version for your camera. They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass, etc. They also enable colors to become more saturated and appear clearer, with better contrast. This effect is often used to increase the contrast and saturation in blue skies and white clouds. Hoya's polarizing filters do not affect the overall color balance of a shot.

How to select the correct polarizing filter:
Many of today's cameras use semi-silvered mirrors or prisms to split the light entering the viewfinder in order to calculate exposure and focusing distance. PL (linear polarizing) filters can sometimes interact with these items to give unpredictable exposure or focusing. So we recommend that you choose a PL-CIR filter unless you have a manual focus camera which has no beam splitter.

Polarizing filters are used to select which light rays enter your camera lens.
Remove unwanted reflections from non-metallic surfaces such as water, glass, etc., in their scene.
Enable colours to become more saturated and appear clearer, with better contrast.
Increase contrast and saturation in blue skies and white clouds.
Dramatically enhance indoor or outdoor photographs.