The Radio-Electronics website has a useful entry on the use of spectrum analyzers for the design, test and maintenance of radio frequency circuitry and equipment.
Spectrum analyzers, like oscilloscopes are a basic tool used for observing signals. However, where oscilloscopes look at signals in the time domain, spectrum analyzers look at signals in the frequency domain. Thus a spectrum analyzer will display the amplitude of signals on the vertical scale, and the frequency of the signals on the horizontal scale.
In view of the way in which a spectrum analyzer displays its output, it is widely used for looking at the spectrum being generated by a source. In this way the levels of spurious signals including harmonics, intermodulation products, noise and other signals can be monitored to discover whether they conform to their required levels.