For a given signal, the power spectrum gives a plot of the portion of a signal's power (energy per unit time) falling within given frequency bins. The most common way of generating a power spectrum is by using a Fourier Transform, but other techniques such as the Maximum Entropy Method can also be used.
Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. ``Power Spectra Estimation Using the FFT''
and ``Power Spectrum Estimation by the Maximum Entropy (All Poles) Method.'' §13.4 and 13.7 in
Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England:
Cambridge University Press, pp. 542-551 and 565-569, 1992.