hmm... guess again.

Overdrive is what happens when the front end of an amp is fed a signal that is strong enough to saturate one or more of the gain stages. The incoming signal can be clean or already distorted; doesn't matter. The point is, that the level is high enough to cause the amp to distort. Usually, an overdrive pedal pushes a strong, clean signal. The resulting distortion is produced within the amp. If the amp has smooth distortion characteristics, you'll get smooth distortion. If you overdrive a transistor amp that was not designed for 'nice' distortion, it won't sound smooth at all.

Distortion pedals deliver a distorted signal. Yeah, really! The output could be turned down so that it was nowhere near strong enough to overdrive an amp, and the sound would still be distorted. This is useful in situations where you can't crank a tube amp up enough to get it to distort on it's own. Or it can add extra crunch when fed into a cranked amp.

There are differences in the sound of distortion generated in a pedal or in the preamp, and distortion generated in the power stage of a tube amp. A distortion pedal can't generate power-stage distortion without also creating preamp distortion. An overdrive pedal will cause the most sensitive stage of the amp to distort. What stage that is, depends on the amp design and the control settings. Which setup you prefer, depends on what you're playing, and what you want it to sound like.