Distortion may bean undesired change in an audio waveform that creates unpleasant sounds or it may be the intentional alteration of a sound signal through electronic amplification, "overdriving" it beyond the system's ability to reproduce the waveform faithfully. Distortion is applied usually and most notably to guitars.
In a typical sustained musical tone, most of the component frequencies are harmonic (related to each other by simple ratios, and thus mutually reinforcing), while others are inharmonic (especially in sounds produced by plucking or striking).
Extreme amplification can lead to clipping of the original signal, which makes the inharmonic partials stronger in proportion to the harmonic ones. Furthermore, if two or more notes are played simultaneously, they give rise to an audible difference tones that further complicate the waveform (e.g., notes at C = 262 Hz and F# = 370 Hz will create a third note at (370 - 262) = 108 Hz = slightly flat A, inharmonic in relation to both original frequencies). High levels of overdrive thus interestingly blur the boundary between timbre and harmony.