If you use an autofocus camera, digital camera or camcorder, a standard 'linear' polarising filter may affect your focus and/or exposure settings. A circular polariser avoids this problem.

A Polarising filter is probably the most useful and least intrusive of all filters. It can be used to remove or reduce reflections off non-metallic surfaces (e.g. when shooting through glass). It can also intensify the colour saturation of a blue sky.
The filter can be rotated to vary the strength of the effect, which will be visible in the viewfinder.