I think overdrive and gain may be the same thing. I think distortion is the effect, but I really don't know for sure. You'd have to ask some of the more tech savvy members.

distortion is a clean signal that has been broken up to distort. gain is a term that refers to taking that distorted signal and increasing it's overall quality with even more of a distorted sound. you often see gain switches on distortion pedals. overdrive is an effect that gives your distorted signal a big kick in the arse and is often accompanied by a slight increse in overall volume. it can also take a clean signal and give it a distorted sound. overdrive pedals are very popular with fender type amps that don't necessarily have a lot of gain to them to begin with. the overdrive pedal can really improve the quality of the distorted sound from the these amps and give them a nice volume boost as well....overdrive is often confused with gain as they have similiar properties and because overdrive CAN add some GAIN to your signal...but there is an actual difference between the two as far as hardware applications are concerned..

the topic gets a little more confusing when discussing the differences between solid state and tube distortion qualities as solid state amps are often designed to provide a rather brittle sounding high gain scenario as opposed to tube amps that generally provide a warmer sounding distortion tone or quality.....

Gain is just that - adding. When you add gain to a tube amp, it pushes [overdrives] tubes, hence "overdrive" pedals try to emulate that sweet overdriven tube amp sound.

Distortion refers to the solid state effect of clipping the electronics to create that distorted tone - sometimes utilizing dual gain stages [cascading gain stages they are sometimes called] to create a supper fuzzy, uber distorted metallic tone.

So that said, the gain knob as you can imagine determines how much signal gets to go through the tubes or whatever they clip in a distortion circuit [diodes???]. Distortion circuits tend to be less linear that a tube circuit - when you roll back the volume knob on your guitar, it cleans up and loses little actual volume because in essence you are changing the amount of gain before it actually gets to the gain knob. Tube amps generally clean up nicer than SS ones, and you will have more dynamic control picking lightly to clean up on an overdriven tube amp than you would picking lightly on a distorted solid state amplifier.

So how far can you distort a signal? What begins to happen at a certain point, I mean what does it sound like when the signal breaks up too much? Also, is "boost" the same thing as overdrive? And by the way, thank you both for this info! I find alot of this stuff confusing, and this is very helpful to me!

You can distort as much as you want but after a point it becomes pointless because you loose definition of the notes you play.
Boost is exactly what the word means. You can boost the volume, overdrive or distortion. It's a less/more switch.

When you use distortion, the sound comes out with "Distortion", if you use your overdrive the sound will came out distorted too, except when your turn down the volume output signal on your guitar (in that case, the sound will be kinda crunchy, istead of distorted).

Well, this is a vertible cornucopia fo knowledge, guys! My tube king has a dial on it, called a VOID dial on it that breaks the signal up. I know it's for the more aggressive metal player, but how does this deal work? Is it plugging the signal, by opening the circuit, or what?