Phase modulation is varying the angle of a wave of the carrier to transmit analog or digital data. For digital signals, phase modulation (PM) is widely used in conjunction with amplitude modulation (AM).

Amplitude Shift Keying (ASK)
ASK modulates the amplitude of the RF carrier on a digital input signal.
Binary 1 represented with some amplitude and binary 0 represented with zero amplitude (no carrier).

Frequency Shift Keying (FSK).
FSK modulates the frequency of the RF carrier on the digital input signal.
Binary 1 and 0 are represented by two different carrier frequencies f1 & f2.

Phase Shift Keying (PSK).
PSK modulates the phase of the RF carrier on the digital input signal.
Binary 1 is represented by 180-degree phase of the carrier and binary 0 is represented by 0-degree phase of the RF carrier.