FDM - Frequency Division Multiplexing

TDM - Time Division Multiplexing

WDM - Wave Division Multiplexing

The Frequency Division Multiplexing is used in a scenario where the bandwidth (in Hz) of a channel is more than the combined bandwidth of the  signals to be transmitted. In FDM, each signal modulates different carrier frequencies which are combined together and sent through a single link.  These channels are separated by a distance of unused frequencies called guard bands.  

The 'Time division multiplexing' is used in scenarios where the channel capacity is less than the combined bandwidth of the signals to be  transmitted. In TDM the channel is divided into fixed time slots instead of frequency as in FDM.

In WDM, high frequency light signals are used to carry data. If a single light signal is used at a time, it will result in the wastage of the available  bandwidth. So, multiple light signals are combined together and sent over the single optical fiber cable. The optical fiber cable offers higher data  rate as compared to metallic transmission cable.