FM is Freqency Modulation

PPM and PCM are ways to encode stick position but PCM is digital, they both use an FM signal to send the information to the reciever. Noise in a PCM signal is rejected as its not a 1 or zero, noise in a PPM signal is amplified and sent to the servos as position information and is whats seen as a "glitch" enough interference and PCM locks out, that is either does nothing or drives the servos to a pre determined fail safe postion (throttle to idle is the main one) until a good frame is recieved then you get control back. The downside is it costs more. Also to use PCM you must have a reciever from the transmitter manufacturer you cant use a generic Hitec reciever with a Futaba transmitter in PCM mode though you could switch it to PPM mode you loose the benefits of PCM

Also most PCM radios can be set on a per model basis to do either PPM or PCM.