Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics which work primarily with analog signals. Despite the name, digital electronics designs includes important analog design considerations.
An industrial digital controller
A binary clock, hand-wired on breadboards
Intel 80486DX2 microprocessor
A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at any given time it represents a real number within a continuous range of values.
A frequency-shift keying (FSK) signal is alternating between two waveforms and allows passband transmission. It is considered a means of digital data transmission.