Multiplexing refers to the ability to transmit data coming from several pairs of equipment (transmitters and receivers) called low-speed channels on a single physical medium (called the high-speed channel).
A multiplexer is the multiplexing device that combines the signals from the transmitters and sends them over the high-speed channel. A demultiplexer is the multiplexing device via which the receivers are connected to the high-speed channel.
Frequency-division multiplexing, also called FDM, makes it possible to share the available frequency band on the high-speed channel by dividing it into a series of narrower-band channels so as to be able to continuously send signals coming from the different low-speed channels over the high-speed channel.
In time-division multiplexing, also called TDM, the signals from the different low-speed channels are sampled and transmitted successively on the high-speed channel by allocating each channel in turn all of the bandwidth, even if it does not have any data to transmit.
Statistical multiplexing is similar to time-division multiplexing except that it only transmits low-speed channels that actually have data on the high-speed channel. The name of this type of multiplexing comes from the fact that the multiplexers base their behavior on statistics concerning the data rate of each low-speed channel.
Since the high-speed line does not transmit the empty channels, performance is better than with time-division multiplexing.