Read about different photographic filters here: http://en.wikipedia.org/wiki/Photographic_filter

A UV Filter is used to reduce haziness created by ultraviolet light, to which photographic film and sensors are sensitive, but not the human eye. A UV filter passes all or most of the visual spectrum, and blocks ultraviolet radiation.

A Neutral Density (ND) Filter is of uniform density which attenuates light of all colors equally. It is used to allow a longer exposure (to create blur) or larger aperture (for selective focus) than otherwise required for correct exposure in the prevailing light conditions, without changing the tonal balance of the photograph.

A Graduated Neutral Density filter is a neutral density filter with different attenuation at different points, typically clear in one half shading into a higher density in the other. It can be used, for example, to photograph a scene with part in deep shadow and part brightly lit, where otherwise either the shadows would have no detail or the highlights would be burnt out.

A Circular Polarizing Filter (CPF), used for both color and black-and-white photography, is colorless and does not affect color balance, but filters out light with a particular direction of polarization. This reduces oblique reflections from non-metallic surfaces, can darken the sky in color photography (in monochrome photography colour filters are more effective), and can saturate the image more by eliminating unwanted reflections.

Linear polarizing filters, while effective, can interfere with metering and auto-focus mechanisms within DSLRs; circular polarizing filters are also effective, but do not upset metering or auto-focus.

Read about Circular Polarizing Filters here:
http://en.wikipedia.org/wiki/Polarizing_filter_%28photography%29
and
http://camerapedia.wikia.com/wiki/Polarizer