The key components of a Wavelength Division Multiplexing (WDM) system include multiplexers, demultiplexers, optical amplifiers, and optical filters. Multiplexers combine multiple data signals onto a single fiber by assigning each signal to a unique wavelength. Demultiplexers separate the combined signals at the receiving end by routing each wavelength to its own output. Optical amplifiers amplify the optical signal to compensate for signal attenuation over long distances, while optical filters are used to isolate specific wavelengths and reduce interference.