Modes
The term mode has varying meanings, according to the context, but the most common are permitted modes in amateur licensing.
A mode is the way of changing electromagnetic waves, modulating them so that transmission of information is possible. There are three "true" modes, or methods of modulation: Amplitude Modulation (AM), Frequency Modulation (FM) and Continuous Wave (CW). Remaining modes are derived from these.
Continuous Wave (CW)
A continuous wave is an electromagnetic wave of constant amplitude and frequency, a pure carrier, and information is carried by turning the wave on and off, and measuring the interval. Morse code is often transmitted using CW.
See http://en.wikipedia.org/wiki/Continuous_wave
Amplitude Modulation (AM)
Where the information is imposed on the amplitude of the carrier. Voice is often transmitted using AM.
See http://en.wikipedia.org/wiki/Amplitude_modulation
Frequency Modulation (FM)
Where the information is imposed on the frequency of the carrier.
See http://en.wikipedia.org/wiki/Frequency_modulation
Digital modes
The licensing regime defines digital modes as those modulation techniques that require digital data processing. Refer to the ACMA LCD for exact details.
Technically, whenever a signal is turned on and off to enable transmission of information, it can be considered to be a digital mode. Under this definition, CW is certainly a digital mode. This section refers to methods of transmitting and receiving (rather than modulating) that are digital, or that require digital processing in part of the transmission or receiving process.