Demultiplexers aren't common here, but every one I have seen has been missing the most important parts, making them just decoders. A demultiplexer _takes_a_signal_ on one line, and sends it to one of several other lines. which one depends on its control inputs. A multiplexer has the exact same decoding logic, but chooses one of several input lines to present as its output.
Also, it's "mux" or "demux" for short.
Edit: Seems actually searching for "multiplexer" turns up better circuits than what I've seen popping up. I'm retaining this as a simple example though. :)
