The information given above by stevebrot is sadly innacurate. To his defense, polarization is one of the most complicated concepts in optics and photonics.
A linear polarizer rejects light polarized along one axis, and transmits light polarized on another axis, erpendicular to the first one. For simplicity's sake, let's keep it simple, and say that light is made of two polarization components, perpendicular to one another. It's a little more complicated than that, but it doesn't need to be, here
After light passes through a LP, it is linearly polarized in one direction. If light was ALREADY polarized (this happens, for instance, when an unpolarized source is reflected on a non-metallic surface) then the polarizer will either
1-let the light pass through, if its orientation is matched to that of the polarizer
2-reject the light, if the polarization orientation is not matched.
3-a combination of both, if the orientation is partially matched
A circular polarizer is a linear polarizer. It does the same thing. the difference is that, AFTER the LP component, there's another component, that will spread the light (which is polarized on ONE axis) over the two polarization axes. To give you an image, let's say light before the LP looks like this : + . After the LP it looks like this : = . After the CP it still looks like this : + . So if you send light oriented like this : | to a LP oriented like this =, nothing will pass. If you send light oriented like this : / , some of it will pass (in this case, the horizontal component).
This may be too complicated, or badly explained, I'll be happy to tell you more (or to say things differently) if needed.
Now, about metering... the thing is that the mirrors inside the body polarize the light themselves, and use the fact that light has two polarization orientations, to send light where it's needed for metering, AF, etc. If the mirrors and polarizers were perfect, metering would probably not work at all. But since those components are NOT perfect, some light can still be sent to the parts of the body that needs them. But remember that it CAN be innacurate in some cases (particularly with wide lenses and in low light). Personally, I'd rather pay for a CP without coatings than a LP with coatings.
My two cents.