There is a difference between a distortion pedal and an overdrive, right? At least that's what I've read over and over again. As Greg and ...oops, forgot, I think Miroslav(?) stated earlier, the overdrive boosts the signal to the amp, so my impression about a distortion pedal is that the sound of the pedal itself is just amplified by the amp. Of course since I'm usually wrong about these technical matters...I just might be wrong about this. I do want an overdrive to keep some of the character of my amp, but don't want just a totally clean boost, a little dirt wouldn't hurt.
Nobody ever addressed this.
I consider overdrive as a verb - the act of hitting a circuit with so much signal that it fails to reproduce it faithfully, pushing the circuit out of its linear operating range. This causes a change in the harmonic content, "adding" new harmonics to the signal. This extra harmonic content is distortion - a noun. If you pump much more than maybe a Volt peak-to-peak into most guitar amps, you will over likely end up overdriving one or more stages, and you will hear distortion as a result.
Course, that ain't the way most people talk about these things, nor how pedals are usually labelled! In general, Overdrive tends to mean "less Distortion". The signal is not pushed too far out of the linear region, and the output is a lot like the input with some added harmonic content. Distortion is when it's taken to extremes. The added harmonics start to overwhelm the original signal, and the output starts to become unrecognizable compared to the input.
In most pedals, this happens inside the pedal itself, no matter the name. A TubeScreamer is not a clean boost except maybe at the lowest gain settings with relatively low input levels. It can be used to overdrive your amp, but there will be some distortion added inside the pedal as well. A Rat (my quintessential "distortion" pedal) can also give an almost clean output, and be used to overdrive an amp. But it will also add distortion on its own. Both pedals have one knob to control how much distortion they add, and another to set the final output level. Interestingly, with everything cranked, you get just about the same Peak-to-Peak Voltage on the output of each, though the Rat will have a higher RMS voltage.
Don't know if any of that helps... The line between Overdrive and Distortion is fuzzy (pun intended) and subjective. There is no standards committee. People call their pedals whatever they want. TubeScreamer-style pedals with the clipping diodes in the feedback path are more often called OverDrive, while pedals like the Rat with diodes to ground are called Distortion, but it's not always the case.
