Quadrature amplitude modulation (QAM) is a method of modulating (encoding) digital signals on a cable television system. It is commonly used to transmit digital cable TV channels on the otherwise analog RF cables, and can be decoded by several types of tuner cards (assuming that the video signal is not encrypted).
QAM conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers.
More information about QAM is available at Wikipedia.