Polarizer filters give you several key creative features. They evenly increase color saturation of all colors (except black and white), creating deep, rich scenics. They add contrast for dramatic skies and arial shots. They eliminate the whitish 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.
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.