Quadrature Phase Shift Keying (QPSK) is a method to transmit digital information on a carrier by changing the phase of the carrier. In QPSK there are 4 different phase changes defined, each phase change represents the transmission of 2 bits. An example of phase change and the bits it represents is given in the table below.
QPSK is used in a number of communication and broadcasting systems, e.g. DVB-S.