FDM, or frequency division multiplexing, and TDM, or time division multiplexing, are both ways of incorporating information into a system. However, frequency division multiplexing uses a system that sends all carrier frequencies separately from one another. By contrast, time division multiplexing sends digital signals that use the same carrier frequencies over the same medium.
According to RFWireless World, multiplexing is the process by which certain types of telecommunications equipment can simultaneously transmit different trains of signals into a single channel. This combines the information contained in the signals into a single system and allows it to be interpreted by other equipment. There are a variety of different methods of multiplexing. There are both analog and digital methods of multiplexing information.
Time division multiplexing is a digital method, though it can have analog applications, while frequency division multiplexing is analog. The key difference between these two methods is in the separation of carrier frequencies. While frequency division multiplexing keeps each of these frequencies separate, time division multiplexing does not. When the frequencies are not separated from one another, the transmission signals can be interleaved. Both of these systems have applications in different industries, such as radio and television. Sometimes they are used in conjunction with one another, depending on their strengths.