Multiplexers (MUX) and Demultiplexers (DEMUX) are electrical circuits that are used it many integrated circuits and so. They use an address control line (usually analog) and control the flow of a data signal. The MUX takes many inputs and has one output. The address line controls which of the many inputs goes to the one output. The DEMux has one input and many outputs. The address line controls which of the many outputs is connected to the one input. In either case, only one of the many inputs/outputs is connected to the one output/input.