A demultiplexer is a type of electrical routing device and is used it both analog and digital circuitry.  For inputs, it has a single signal line and for outputs it has two or more signal lines.  Its purpose is to route a single input signal into a particular output channel.  The channel is determined by selector inputs, which represent a unique output channel.

Demuxers are often used in conjunction with multiplexers to provide a routing methodology, which saves space in a layout when there are a high amount of data channels.  A mux selects from multiple inputs and route that to a single output.  That output then goes to the input of the demux and be sent to a specific, selected output.  In many cases, this saves a considerable amount of space. 

A decoder is a device, which activates an output channel unique to a particular logical value applied to the selection lines.  Demuxers can also be used as a decoder device for digital inputs.  If the signal of entry is always a logical 1, then the selection lines can be used to route that value through to the appropriate output line.  This type of decoder device is called a 1 to n device.  Only one output channel at any specific time is routed to the input.