Modulation may be either analog (continuous) or digital (transmission of a finite number of discrete states).

Common types of modulation include:

AM: amplitude modulation - information is passed by varying the signal strength

QAM: Quadrature Amplitude Modulation

FM: frequency modulation - information is passed by varying the signal frequency

FSK: Frequency Shift Key [example system: CT-2]
MSK: Minimum Shift Key
GMSK: Gaussian filtered Minimal Shift Key [example system: GSM Cellular]

PM: phase modulation - information is passed by varying the phase of the signal

BPSK: Bi-Phase Shift Key varies between 2 phase angles 180 degrees apart. [example system: many low cost ISM transceivers]
QPSK: Quadra-Phase Shift Key varies between 4 states at 90 degree spacing.
/4 DQPSK: Pi by four differential QPSK- QPSK rotated 45 degrees [example systems: IS-54 (TDMA) US Cellular, Japan Cellular]
ODQPSK: Offset Differential QPSK - QPSK alternately rotated zero and 45 degrees. [example systems: base station of IS-95 (CDMA) US Cellular]

m-ary modulation varies both amplitude and phase to produce m states. Common m-ary schemes include quaternary (4-ary), 16-ary, and 64-ary. [example systems: Point-to-Point communications links; Digital Cable TV]