A demux is a digital switch that takes a single input signal and distributes it to multiple outputs based on select lines. It has one input and multiple outputs, with the select lines determining which output the input is routed to. Common demux configurations include 1-to-2, 1-to-4, 1-to-8, and 1-to-16 depending on the number of select lines. A typical application is routing a single data source like a laser printer to multiple destinations like a fax machine, inkjet printer, or plotter.