Harmonic series.. so essentially every sound has a frequency which is a wavelength, the higher the note the shorter the wavelength and the shorter the frequency. Each instrument creates a different sound due to its structure which creates a different wavelength and frequency even though they all may be hitting the same middle c.

A harmonic series can have any note as its fundamental wavelength. So there are many different harmonic series but the relationship between frequencies in the series are the same. So essentially say you have a trumpet on a middle C, that creates a fundamental wavelength...lets call it the "Fundamental note"..or the "Fundamental Harmonic"... the second harmonic always has exactly half the wavelength (twice the freq) of the fundamental; the third harmonic always has exactly a third of the wavelength (3 times the freq) of the fundamental.. etc..

Thats essentially it. From what i understand

-Tim