As opposed to overdrive (which is generally a smooth, more organic sound of a cranked guitar amp), distortion refers to the more radical means of simulating high gain amplifiers. To use an analogy, an overdrive pedal is like a cranked Fender Bassman or early Marshall. A distortion pedal strives for more high-gain sounds such as a Mesa/Boogie, Marshall JCM800, or Bogner. In addition to boosting the guitar signal, the actual waveform of the signal is altered, resulting in a - you guessed it - distorted sound. Distortion pedals are also good for obtaining hi-gain sounds at lower volumes. Players from a wide variety of genres use distortion pedals, e.g. both Jeff Beck (a rock guitarist) and John Scofield (a jazz guitarist) use a ProCo RAT distortion.