So some of the most popular effects for the guitar are overdrive and distortion. There is some gray area about what the differences between the two are and there's a lot of room for debate. There's a lot of crossover, where you'll have pedals that could be classified as both distortion or overdrive.
But as far as where overdrive comes from and what it is, I'd say in the 50s', as early as artists like Link Wray who is somebody that I believe to be one of the first people to use overdrive, it wasn't in a pedal. What it was was cranking tube amps. The effect of overdrive comes from cranking an amp.
So now we have pedals which are made to give you the impression that you're cranking your amp at a low volume. It can also be at a high volume too. But in pedal form, what you're emulating is the sound of a cranked amp. So the two pedals that I have here are two of my personal favorites, the Fulltone OCD pedal. He has about four or five different versions now.
This is an early one, a version one. It has the white knobs. You can't get it anymore. I think maybe on eBay. This one is a great pedal. It's a must-have in my arsenal and next to it is a classic TS-9 Tube Screamer Pedal. This one was modified with a chip called the brown chip, which is emulating an even earlier version of this pedal which was the TS-808, which is a very classic overdrive pedal.
So this one sounds as close to the TS-808 as you can get. Still the TS-9 is a very classic pedal. I personally like a very organic sound. I like the sound of a cranked amp. In most situations, you can't have an amp cranked at full volume, so these two overdrive pedals are the ones I tend to use, because I feel like they're the most transparent.
What I mean by transparent is I mean that they get the true sound of your guitar, the true sound of your amp to sound like the amp is cranked at low volume, without adding much color. And when you add color you get into distortion territory and I'm not a heavy distortion user. I like overdrive and I like a very natural sound, so to me, these are the two pedals that I personally like to use the most.