At the risk of scaring more people off, I will briefly explain what Fourier Transforms (FTs) "do." I'll use sound for this example, since that is what the end goal in this case is. Basically, when you look at a sound - which is caused by something vibrating - you usually look at the movement of the vibrating stuff as it varies through time. Alternatively, you could also look at what 'amount' of which frequencies of vibration are present in that sound. A FT is a mathematical mechanism of switching between those two views.
If you have a recorded audio signal, which is a measurement of the vibration, and you apply a FT to that signal, you will get a measure of the frequencies in the audio, instead. (As a side note, in Quantum Mechanics, FTs are often used to switch between a momentum distribution and a position distribution.) A FFT is simply "an efficient algorithm to compute" the FT - in other words, it's faster than doing a FT directly.
Take that WYUR!
[PS. Audacity is a really great FREE! program]