When light from the sun bounces off a flat non metallic surface, such as glass or water, it becomes polarised with all of the reflected light waves vibrating in the same plane. This reflected glare can be removed by the polarising filter.
There are two types of polariser, circular and linear. These names bear no relation to the physical shape of the filter itself but refer instead to the way the light waves are transmitted through the filter to achieve the polarising effect.
Users with an autofocus SLR should use a circular polariser.
Those with a manual focus camera, either 35mm or medium format, can use either a circular or linear polariser.