What do they do? Polarizers are designed to block light from a certain direction so depending on the situation, you rotate the polarizer in front of the lens until it eliminates the unwanted light. This is not the same as an ND filter that simply blocks light. Just get the right size for your lens and instantly add more contrast to your images as you shoot.
I use the B+W 77mm Kaesemann Circular Polarizer and can be found here.