Polarization filters are handy tools, when used right. They reduce or eliminate certain reflections on non-metallic surfaces and so enhance the colors of the objects:
But sometimes reflections are somewhat essential. When they're removed completely, the image simply looks boring:
This works in big scale too:
Both images were taken immediately in a row near sunset. The differences are clearly visible. Basically the filter reduces reflexions from water-molecules in the air, in this situation we got a decent increase in contrast.
There are two kinds of polarization filters, linear and circular. The difference is how the light leaves the filter in the camera's direction. Especially SLR cameras contain a semireflective mirror to separate some light for the metering system. These mirrors are sensitive to lineary polarized light and may produce badly exposed images in combination with a linear polarization filter. You should read your camera's manual to verify, if you need a circular polarization filter.