A: The terms are often used interchangeably, so providing more than a vaguely general answer is difficult. However, in our opinion, with guitars “overdrive” refers to the sound of an amplifier that has been pushed past where it is capable of producing clean tone – whether that “push” is coming from a pedal or the amp itself. (However, many people would also say the amp has been pushed into distortion…so there you go; vague generalities…)
“Distortion,” with guitar amps and effects — and again, this is our opinion — is often used to refer both to “fuzz” types of effects as well as overdrive types of sounds. It’s a more general term that can be applied to any audio signal that is not clean.
As a final note, for some guitarists, the difference between overdrive and distortion is a matter of degree. A lightly distorted sound or small amount of breakup is refered to as “overdrive,” while a more heavily distorted sound is refered to as “distortion.”