They are digital switches which connect data from one input source to one of n outputs.
Usually implemented by using n-to-2n binary decoders where the decoder enable line is used for data input of the de-multiplexer
The figure below shows a de-multiplexer block diagram which has got s-bits-wide select input, one b-bits-wide data input and n b-bits-wide outputs.
Mechanical Equivalent of a De-Multiplexer
The operation of a de-multiplexer can be better explained using a mechanical switch as shown in the figure below. This rotary switch can touch any of the outputs, which is connected to the input. As you can see at any given point of time only one output gets connected to input.
1-bit 4-output de-multiplexer using a 2x4 binary decoder.
Example: 1-to-4 De-multiplexer
Mux-Demux Application Example
This enables sharing a single communication line among a number of devices. At any time, only one source and one destination can use the communication line.