Polarizer filters evenly increase color saturation of all colors (except black and white), creating deep, rich scenics. They add contrast for dramatic skies and arial shots. Polarizers also eliminate glare from highly reflected surfaces like glass and water. Polarizers are two part lenses that rotate, allowing you to select the degree of additional saturation, contrast and glare reduction desired.
They come in two varieties. Circular-for all types of cameras (required for beam splitting metering systems commonly found in auto-focus SLR's and in most current TTL SLR's; check your camera's manual). Linear-for rangefinders and older SLR metering systems.