PWM - Pulse Width Modulation
PWM has been around for a long time in the radio control hobby and is the protocol that your ESC’s and servos talk to each other on. The analogue signal takes the form of a pulse, and the length of the pulse represents a specific value, with 1000 being minimum, and 2000 being the maximum value.
Each channel has its own single wire, so if you have an 8 channel receiver you will need to connect 8 wires to read the inputs into your flight controller.
Almost every receiver you can buy will have the PWM output as it is very popular and generally supported by all flight controllers so its a good safe option, however the fact that you need to connect a wire per channel can get abit annoying and possibly lead to mistakes if you get the numbering incorrect.
PPM - Pulse Position Modulation
PPM is also an analogue signal, but instead of using a separate wire for each channel, PPM stacks each signal one after another to send them all along the same wire. This makes wiring your R/C Receiver to your autopilot much easier! Other variations of PPM include CPPM, and PPMsum which are slight variations introduced by specific manufacturers.