Distortion

Distortion is processing of the guitar signal to produce that sound. This won't likely come as a shock, but the device used for this is called a signal processor or stomp box. Ironically, and despite the name, distortion is purely an act of precise refinement; that is to say it is an exact science and not just poor sound quality. Originally discovered in the mid 20th century by manipulation of the mixing knobs on an amp to produce the effect, distortion has evolved far. Grunge or pop rock may have light twangy distortion whereas heavier rock and metal will have deep powerful distortion. Distortion is necessary in order to perform some techniques such as harmonics and squealing. There are many other kinds of signal processing for different styles but the most staple is probably distortion. Not to be confused with active effects such as pitch bending or wah-wah which are consciously activated with and expression pedal.