Tiffen says at http://www.tiffen.com/polarizer_pics.htm
Originally Posted by Sailorlite
Certain camera optical systems employ internal surfaces that themselves polarize light. Using a standard (linear) polarizer will cause the light to be further absorbed by the internal optics, depending on the relative orientation. A Circular Polarizer is a linear one to which has been added,on the side facing the camera, a quarter wave "retarder." This "corkscrews" the plane of polarization, effectively depolarizing it, eliminating the problem. The Circular Polarizer otherwise functions in the same manner.
Yeah, that doesn't mean much to me, either.