Watch the Distortion online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide
A distortion box produces what we call hard clipping, as opposed to an overdrive pedal that produces soft clipping. What does that mean? If we were to look at a note on an oscilloscope we would see a wave (hence the term sound wave) with a nice rounded top and bottom. When we overdrive or distort a note the top and bottom of the sound wave flattens out, or clips. The more clipping of the wave the more distorted the note. A fuzz pedal produces so much clipping it is seen as a square wave, completely flat on the top and bottom of the wave. There are many excellent distortion boxes on the market but some classic industry standards are the Rat distortion box, the Boss Super Overdrive (a bit more of a distortion box really), MXR Distortion +, Boss Distortion, and the Boss Heavy Metal Pedal.
Many distortion pedals can also be used as overdrive pedals simply by reducing the gain, so once again we see how these terms are a little loose. In high gain amps like a Mesa rectifier the amp is taking advantage of gain staging, many pedals do this as well. Gain staging is simply putting one overdriven tone into another and cascading them to produce even more gain or distortion. So in a Mesa, one preamp tube is being run into another to bump up the level of distortion, there can be any number of gain stages. We can also do this by stacking pedals as well, as we will see in the gain staging pedal chain section. Dialing in a good distorted tone can take some time and slight EQ changes can make a big difference.