Modulation: YOu take an input like a binary input for example. When you modulate it, you convert the binary bit stream to a waveform. When you demodulate the received waveform, you convert it back to a bitstream.
1.Demodulation: The recovery, from a modulated carrier, of a signal having substantially the same characteristics as the original modulating signal.
2.Demodulation - To recover the signal from the carrier. For example, in a radio broadcast using amplitude modulation the audio signal is transmitted as the mean amplitude of a radio-frequency carrier so demodulation requires a circuit which measures the amplitude and filters out the carrier
Demodulation is the process of recovering the information signals from the carrier, which was employed in order to make the signal robust against propagation problems.