Any periodic (or almost periodic) signal is absolutely equivalent to the superposition (the sum) of sine (harmonic is probably more correct) signals called fundamental and harmonics; the fundamental component has the same frequency as the complete signal and the others have frequencies multiple of this one. The harmonic 2 has a frequency double of the fundamental, the harmonic 3, triple, ... The series of fundamental and harmonics is called Fourier series. The web page http://en.wikipedia.org/wiki/Fourier_series gives drawings of interesting examples, though the article is too detailed and mathematical. A practical example is the signal of a relaxation oscillator. A band-pass filter connected to the oscillator and tuned to the fundamental frequency or to one of the harmonics allows to extract the corresponding sine component. Reciprocally, it is possible to reconstruct a complicated periodic signal in summing its harmonic components, using, for example, an op amp adder. However, I doubt that this has many useful application, contrary to the filtering of complex signals.
