Quadrature Phase-Shift Keying
is a sibling of the Phase-Shift Keying
QPSK is a form of modulation in which the carrier signal is sent in four phases, 45°, 135°, 225°, and 315°. The change between one phase and the next encodes 2 bits per symbol. With a total of 4 symbols at a 90° seperation, this allows for 8 bits per signal.
QPSK is seen as providing the best error rate in the presence of radio noise. It's most common in IEEE 802.11b wireless equipment and satellite equipment.