What is the differance in PCM and PPM I have a Airtronic radio that has a switch on that saids PPM - PCM.

They are, in very basic terms, two different forms of modulation transmitted to the reciever. If you really want to know the details....and it's very involved... here's a link that will explain it fairly well- PCM vs. PPM
Here's another site that explains it in a lot more depth.
For practical purposes, you only need to know that if you're using a PPM reciever, switch the transmitter to PPM and visa versa for PCM.