Overdrive - Distortion - Fuzz Effect

Distortion, overdrive or fuzz, is an effect applied to the electric guitar, the electric bass, and other amplified instruments such as the Hammond organ, synthesizers, and even harmonica and vocals.
Distortion can be produced by many components of an instrument's signal path, including effects pedals, the pre-amplifier, power amplifier, speakers, or more recently, digital amplifier modelling devices and software. Many players use a combination of these to obtain their "signature" tone.
Accomplished by electronically compressing and/or clipping the input signal, this effect adds sustain and additional harmonics and overtones to the signal, creating a richer sound. The most subtle types of distortion add a "warm" thickness to the original tone; the more extreme types of distortion range from the noisy, buzzy sound of a late 1960s-era fuzztone pedal to the screaming, "bite", "grit",and "crunch" of a late 1980s thrash-style distortion pedal. Distortion is used across a wide range of musical genres, from the subtle overdrive used in traditional blues to the hard-edged distortion featured in noise music, hardcore punk, grunge, and metal.

Overdrive/distortion pedals
Because they are often designed to operate off of low voltages such as a 9 volt battery, overdrive and distortion pedals typically use transistors to generate distortion. Classic examples include the Ibanez Tube Screamer and the Electro-Harmonix Big Muff. A few more modern effects pedals incorporate valves, but usually these still run at voltages that are too low for the valve, resulting in a "starved plate" configuration that generates harsh and buzzy distortion. Distortion pedals usually also provide signal gain, which can be used to drive the input stage of the pre-amplifier harder, resulting in further distortion and, in some cases, higher volume
Pre-amplifier distortion
The pre-amplifier section of a guitar amplifier serves to amplify a weak instrument signal to a level that can drive the power amplifier. It often also contains circuitry to shape the tone of the instrument, including equalization and gain controls. Often multiple cascading gain/clipping stages are employed to generate distortion. Because the first component in a valve amplifier is a valve gain stage, the output level of the preceding elements of the signal chain has a strong influence on the distortion created by that stage. The output level of the guitar's pickups, the setting of the guitar's volume knob, how hard the strings are plucked, and the use of volume-boosting effects pedals can drive this stage harder and create more distortion.
Power amplifier distortion
Power valves can be overdriven in the same way that pre-amplifier valves can, but because these valves are designed to output more power, the distortion and character they add to the guitar's tone is unique. During the 1960s to early 1970s, distortion was primarily created by overdriving the power valves. Because they have become accustomed to this sound, many guitar players favour this type of distortion, and thus set their amps to maximum levels in order to drive the power section hard. Because driving the power valves this hard also means maximum volume, many solutions have emerged that in some way divert some of this power valve output from the speakers, allow the player to generate power valve distortion without excessive volume. These include built-in or separate power attenuators and power-supply-based power attenuation. Lower-power valve amps (such as a quarter-watt or less), speaker isolation boxes, and low-efficiency guitar speakers are also used to tame the volume.