A fast Fourier transform is an efficient algorithm for working out the discrete Fourier transform - which itself is a Fourier transform on 'discrete' data, such as might be held on a computer. Contrast this to a 'continuous Fourier transform' on, say, a curve. One would need an infinite amount of data points to truly represent a curve, something that cannot be done with a computer Check out: The Scientist And Engineer's Guide To Digital Signal Processing It is a free, downloadable book that deals inter alia with Fourier transforms chapters 8-12 are germane to your question.
This is a highly practical, roll-yer-sleeves-up book for, as the title says, scientists and engineers, but Smith describes the underlying theory well. The sample code supplied with the book is in BASIC and FORTRAN, of all things; the author does this for didactic purposes to make the examples easy to understand rather than efficient.
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