Multiplexer is a communications device that multiplexes (combines)  multiple signals for transmission across single transmission medium. It  is also called MUX. 
- A demultiplexer completes the process by separating multiplexed signals from a transmission line. It is also called DEMUX.
 
- Frequently a multiplexer and demultiplexer are combined into a single device capable of processing both outgoing and incoming signals.