The circular polarizer filter produces the same effect as a linear polarizer filter, but is used in cameras where polarizing elements are used in the beam-splitters of the camera. If using a linear polarizer with such cameras, the light can become cross polarized (giving the same effect as using two polarizers together).
Circular polarizers are made from a different polarizing material than linear and will usually prevent the cross polarization.
Note that circular polarizers may only be used one way in front of the lens and are marked accordingly.
For more information on polarizers, please see this
polarizers technical data PDF