Frequency Division Multiplexing or FDM is a method of multiplexing. By utilizing different frequencies several channels can be combined in to a single transmission. This is done by assigning each channel a differnt and unique separate part of the frequency spectrum. This is called the carrier frequency. Not ot all possible the frequencies are used. Nonused frequencies or "guard bands" are used to prevent interference between the channels. A good example is television transmitters. They use it to send several channels at once.