Distortion effects create "warm", "dirty", and "fuzzy" sounds by compressing the peaks of a musical instrument's sound wave and adding overtones. Distortion effects are sometimes called "gain" effects, as distorted guitar sounds were first achieved by over-driving tube amplifiers. Distortion has long been integral to the sound of rock and roll music, and is important to other music genres such as electric blues and jazz.
The terms "distortion", "overdrive" and "fuzz" are often used interchangeably, but they have subtle differences in meaning. Overdrive effects are the mildest of the three, producing "warm" overtones at quieter volumes and harsher distortion as gain is increased. A "distortion" effect produces approximately the same amount of distortion at any volume, and its sound alterations are much more pronounced and intense. A fuzzbox (or "fuzz box”) alters an audio signal until it is nearly a square wave and adds complex overtones by way of a frequency multiplier.
Distortion can be produced by effects pedals, rackmounts, pre-amplifiers, power amplifiers, speakers and more recently, digital amplifier modeling devices and software