I need an "Overdrive 101"

I've used distortion stomp boxes, but I hate to say am completely clueless as too how you use Overdrives and what exactly their capabilities are. I've heard that people use them to get light blues distortion, or to turn a mild gain Marshall into a high-gain monster. Can someone give me a crash course as too what are the best Overdrives with certain amps and which settings produce a particular type of sound.

Wow, no short answer to that. I'm going to say that nothing is better than first hand experience. Overdrives range in style and expense so much much that it is difficult to summarize in a single post. My advice would be to just go try some out and if you don't want to spend a ton of cash just by something basic, like an Ibanez TS-9 re-issue or even a Digitech Bad Monkey (gets rave revues and it cost $39). That would give you an experience with them and you can see if they fit for you. They generally push the upper midrange frequencies a bit and can give some bight and snarl to your sound. They are also capable of pushing the front end of your amp into distortion by backing down the gain and using it as a clean boost. I hope that helps.

I agree..trying things is the best advice.
But the idea is...an overdrive can give you more gain and tone than not having it there. If used well, it can push the input harder and give you more distortion and some more volume. It can also allow you in some cases to turn down your guitar volume and clean things up more than you could before.
Your mileage WILL vary...but it starts with trying things and seeing what works.
Tip: do this at volumes just like you are playing in a live band. A teeny weeny volume doesnt reveal what you're actually getting.
-GtrGeorge