Quadrature amplitude modulation
Quadrature amplitude modulation (QAM) is a modulation scheme where both phase and amplitude are modulated in order to encode each symbol. The attached number specifies how many different states can get encoded (e.g. QAM-64 encodes 64 states in the Phase Diagram, QAM-256 encodes 256 states). The following pictures give you an idea what happens:
The more states are encoded for a given allowed signal bandwidth the more susceptible the signal is to noise. On the other hand, the more states are possible the more bandwidth is in theory possible. But only if the signal quality is good enough to decide between the different states because amplitude noise and phase noise will disturb the signal.
US broadcasters use 8VSB modulation.