What are analog and digital signals?
Often, people will think that any signal that switches between two voltages is "digital". This is not correct.

"Digital" means that the signal is carrying information in the form of discrete symbols.  As long as the symbols are received correctly, no information is ever lost in transmitting a digital signal.

Analog signals are always degraded by transmission, even if the degradation is very small.


PCM signals are digital, and consist of switching between two voltages.  As long as you receive the series of 1s and 0s in order, no information is lost.
PWM signals are analog, and consist of switching between two voltages.  The exact timing of the pulses carries information, so if there's any variation in timing from noise, etc. the signal is corrupted.
FSK signals are digital, and consist of sine waves.  As long as the frequency of each bit is recovered, no information is lost.