Modulation is the process by which a carrier wave is altered in such a way that it is able to carry information on it. There are three basic methods to do this:
The primary purpose of modulation in a communication system is to generate a modulated signal suited to the characteristics of a transmission channel. In radio communications, modulation is needed in the transmission systems to transfer the message into the available high frequency radio channel. Modulation is also used in e.g. voice-band modems where digital data modulate the carrier frequencies inside the voice frequency band.
If the message is digital, the term keying is used. So in digital communications the terms Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) are used.