The Fourier transform of a pure sinusoidal signal from the time domain to the frequency domain results in a single spectral line. All other non-sinusoidal signals such as square wave signals, modulated signals, pulses, etc. consist of a fundamental wave and several harmonics differing in amplitude.
In computer technology, the Fourier transform is usually implemented as a mathematical procedure in the form of the Fast Fourier Transform( FFT). The inverse of the Fourier transform, i.e. the transformation of a frequency-related function into a time-related one, is called inverse Fourier transform. The name of the transformation goes back to the French mathematician Baron Jean Baptiste Joseph Fourier, who lived from 1768 to 1830.