The Polarizer is one of the most used add-on filters. Its main use is to kill the reflection form the Sun or other light source. It can also be uses as a NDF if needed.
The filter is attached to the camera and then the outer ring is rotated to achive the best effect.
There are 2 types of polarizers, Circular and LINEAR. From what I can read and understand there is really no difference in them for digital cameras.

This link will show you just how a Polarizer can help your picture.
http://www.pbase.com/otfchallenge/use_of_polarizer_lens

By Bill Huber