You can still buy bindings such as the P18. The main difference is the heelpiece; the pivot series had something of a turntable where the heelpiece screwed into the ski, so that some side to side motion was allowed. In theory, this allowed a little extra time before the binding released (to avoid pre-releasing). This led to some uncontrolled side to side motion of the ski, especially towards the tail (but not much). Additionally, the close screw pattern could pull out of some weaker skis, but on skis with an adequate binding plate, it disrupted the natural flex less than most other bindings on the market.
The PX series has a longer screw footprint and loses the turntable. This means that side to side stability is better than the pivot series, and the bindings are less likely to pull out of weak skis, but it causes more interference with the natural flex of the ski.
There's a quick guide, correct me if I am wrong.