Light rays which are reflected by any surface become polarised and polarising filters are used to select which light rays enter your camara lens. PL (Linear Polarising) and PL-CIR (Circular Polarising) filters have the same effect but it is important that you choose the correct version for your camera. They allow you to remove unwanted reflections from non-metallic surfaces such as water glass etc.
They also enable colors to become more saturated and appear clearer with better contrast. This effect is often used to increase the contrast and saturation in blue skies and white clouds. PL (Linear Polarising) filters can sometimes interact with these items to give unpredictable exposure or focusing.
So we recommend that you choose a PL-CIR filter unless you have a manual focus camera which has no beam splitter.