P12 and P14 are up to 2004/05 model year Pivot model bindings by Look. The P12/P14 have plastic housings with some metal and are more than adequate for most people (I have a pair of P12s, I weight 200 pounds and they are fine at 10). P18 is the race version of the binding which has a true turntable heel and a DIN of 8-18. All metal, very solid binding.
PX is the new Look binding which doesn't have the turntable/axial heelpiece. Most people prefer last years. I haven't tried the PX so I can't comment, Look claims it is technically better but the market concensus is that they switched the design to save on manufacturing costs and to have a binding with a solid "click-in" feel like Salomon (stupid but true).