A polarizer cuts glare, which gives the effect of darkening blue skies, and saturating colors. Foliage particular becomes a much lusher looking green, rather than the bluish green it looks like without the pol filter under bright, sunny skies. Although the effect is less noticable, it also works great on overcast days.
A pol filter may not be good in all situations. The outer ring of the pol filter must be rotated to get the effect. You can see the polarizing effect come and go as you rotate. Some filters have a mark that if lined up with the sun will provide the max effect, but it's usually easy enough to see it through the viewfinder.
You will get maximum polarizing effect outdoors if your lens axis is perpendicular to the arc of travel for the sun. In other words if you stick your arms directly out to either side of your body, and rotate your body so that one hand (either one) is pointing where the sun rises, and the other at where it sets, if you point your camera in the direction your nose is pointing you will get maximum polarizing effect (with the proper rotation of the outer ring). The more you turn so that you are facing where the sun rises or sets, the less effect you'll get. With wide angle lenses you can sometimes see the variation in polarizing effect as it lessens. This is easy to see in the sky where one side of the photo the sky is almost navy, and it fades to cyan at the other side.
You get max glare reduction on reflective surfaces if the lens axis is angled at about 30 degrees to the reflective surface.
Polarizers block 1.5 to 2 stops of light.
