Polarizing filters are cheap but yet so important. They are simple to use, yet pretty hard to explain. This video to show you a few examples how to use them.
The polarizing filter used with most modern cameras is a circular polarizer. The first stage of the polarizer is a linear filter which filters out light that is linearly polarized in a specific direction. The second stage, for technical reasons related to the auto sensors within the camera, then circularly polarizes the light before it enters the camera.
The polarizing filter has two applications in both color photography and black-and-white photography: it reduces reflections from some surfaces, and it can darken the sky.
Use of a polarizing filter, in the correct direction, will filter out the polarized component of skylight, darkening the sky; the landscape below it, and clouds, will be less affected, giving a photograph with a darker and more dramatic sky, and emphasizing the clouds. Perpendicularly incident light waves tend to reduce clarity and saturation of certain colors, which increases haziness. The polarizing lens effectively absorbs these light waves, rendering outdoor scenes crisper with deeper color tones in subject matter such as blue skies, bodies of water and foliage.
The benefits of polarizing filters are largely unaffected by the move to digital photography: while software post-processing can simulate many other types of filter, a photograph does not record the degree of polarization, so the optical effects of controlling polarization at the time of exposure cannot be replicated in software.
Source : Wikipedia