This technique is used, for example, to provide fifty CATV channels on one coaxial cable; or to provide Internet access over cable TV; or to add DSL to a voice-grade telephone line.
A common technique is frequency-division. Each channel is modulated to a different frequency band and combined in the transmission medium. It is demodulated to its original frequency at the receiving end. Channels are separated by guardbands (empty spaces) to ensure that each channel will not interfere with its neighboring channels.