The simplest method of PSK, also called biphase modulation, uses two signal phases - 0 degrees and 180 degrees. The digital signal is broken up according to time into binary digits and the state (1 or 0) of each bit is determined according the state of the bit that preceded it. If the phase of the bit does not change then the state of the signals stays the same. If the phase of the signal changes by 180 degrees, then the signal state changes (from 0 to 1, or 1 to 0).
There are more complex forms of PSK that rely on four or eight phases to transmit data at a faster rate.
Compare with FSK.