Digital Communication is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel.

Under Digital Communication, the data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method. The passband modulation and corresponding demodulation (also known as detection) is carried out by modem equipment, most commonly.

Source: Wikipedia.