In its most basic form, a spectrum analyzer is a testing tool that evaluates a variety of circuit or system characteristics in the radio frequency range. Standard testing equipment would evaluate the quantity by calculating its amplitude over a certain period. It is also called a frequency analyzer.
Voltmeters, for instance, use the time domain to measure voltage amplitude. We may thus expect a sinusoidal curve for alternating current voltage and a straight line for direct current voltage. On the other hand, Spectrum analyzers would evaluate the amount by plotting its amplitude against its frequency.