This is a relatively hot topic of conversation for beginning guitarists, and most articles online provide a lot of fluff to say "Distortion is edgier. Overdrive is natural. Fuzz is fuzzy." Some show sine waves being processed through different pedals. And how does distortion from an amp play into this? Or clipping on recording equipment? Or distortion in an analog echo regeneration?
I think this topic is a lot more subjective than "What's the difference between a flanger and phaser?". There is a specific answer to the flanger vs phaser question. There isn't for this. Sure, a Boss Metal Zone pedal probably won't be mistaken for an overdrive pedal, but as far as most pedals are concerned, the answers aren't black and white.
First, and most importantly, this isn't that important. Musicians and engineers discuss this sort of thing far less the longer they've been around, and the terms become almost interchangeable amongst the most famous musicians I've worked with. Never-the-less, the sounds these pedals make are still wildly different, and it's not a bad idea to be aware of some of the sonic elements you'll be listening for when searching for your own tone.
It would be super convenient to show some waveforms of different pedals processing sine waves right now, except that they don't tell us almost anything. Beyond the basic "all three of these effects distort the sine wave, often into something slightly closer to a square wave" observation, the way these pedals handle a uniform sine wave tells us practically nothing about their behavior. The electrical signal that comes off your guitar is a lot more complex than a sine wave. Even a single note has lots of different sine waves in it, which all decay at different rates. The sound of your pick hitting the string or the strings rattling against the frets is another set of completely unrelated waveforms, of a much higher frequency.
Then hit two notes at once- another totally different ballgame. You not only deal with the behavior of two separate sets of these waves, but you deal with the resultant interaction of all those waveforms, which creates a third set of waves. (This is called heterodyning. A great way to hear how a pedal handles this is to play a B on the 12th fret of your B string and an A on the 14th fret of your G string in the same pick sweep and slowly bend the A up a whole step to a B. If you listen carefully, while the A is rising to an B, you'll hear a third note dropping in pitch until it turns into a tremolo-like pulsing that slows down as the two notes converge.) Different pedals will appear to your ears to handle heterodyning in different ways too, depending on if you've played something like a perfect fourth or perfect fifth, or if you've played something like a second or third.
There's more weird shenanigans like this too, but the point is, different pedals, even within the same Overdrive/Distortion/Fuzz category, handle these situations very differently. Some people like mentioning "Soft Clipping" and "Hard Clipping" as ways of describing these behaviors, but that's like describing all food flavors as spicy or bland. Audio signals are clipped when a component is either unable to reproduce the requested signal or the component is supposed to be non-linear. In the former case, an input signal may go into an op-amp that is supposed to double the voltage of the signal, but at a certain point, it can't double the voltage because the output would exceed the supply voltage to the chip, so it does the best it can on the output until it tops out at the supply voltage. That's one way to clip a signal. Each different type of op-amp will behave differently in that scenario. Or maybe instead of making your input too high, you lower the supply voltage, like how the Choke knob on the Century of Progress Distortion lowers the voltage on a TL072 op amp, which creates a totally different type of distortion than the Drive knob does. There's more ways to get an op amp to clip a signal too, and there's lots of different op amps. Plus, that's just one component. Bipolar Junction Transistors, Field Effect Transistors, Diodes, Vacuum Tubes, and a lot of other components all distort signals in different ways.
Kinda getting lost? Good. The point is, it's not that black and white from a technical standpoint. Resistors, capacitors, and inductors are some of the only components that aren't used directly for distorting audio (although some people will argue with me about multiple things in that statement) but chances are that most other components are capable of making some type of distortion.
Some qualities to think about with each new overdrive/distortion/fuzz pedal you find:
- How does the attack of your pick sound with the pedal?
- Is there a natural sustain to the pedal, or is it a fuzzy sort of sound with very little contour?
- How does it handle something with a lot of treble, like a D chord, strummed near the bridge?
- How does it handle something with a lot of bass and fundamentals, like an E chord, strummed over the fretboard?
- In addition to how it distorts those frequencies, how much low or high end comes through?
- How does it handle palm muting?
- How does it handle the A->B heterodyning trick mentioned above?
- How does it handle pull offs and hammer ons?
- How does it handle consonant sounds, like a power chord or an octave?
- How does it handle dissonant sounds, like playing the fourth fret of the B string with the open high E string?
- Does it ever self oscillate? (This can be a good or a bad thing!)
- How much distortion can you get? How little?
- How much tonal control does the pedal give you?
- Is there one sweet spot with the settings, or can the pedal function to provide multiple usable sounds?
- MOST IMPORTANTLY - HOW FUN IS IT TO PLAY?