The QPSK is the most robust modulation. It can be represented by a constellation:

The radius, R, represents the amplitude.

The angle, φ, represents the phase.

There are 1 amplitude but 4 phases to 4 different states.

2 bits can be coded with 1 QPSK symbol.

The 16-QAM can modulate 4 bits per symbol.

The 64-QAM can modulate 6 bits per symbol.