A multiplexer:

Is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal.

Is a circuit that is able to have inputs of data from a number of sources and then, by selecting an input channel, give an output from just one of them.

Is a system used for taking data from sensors and inputting that data into a computer for processing.

Is used for collecting the parity bits for validating the transfer of information.