Overdrive pedals add grit to the signal, making it sound like a tube amp at its maximum volume.
Distortion pedals provide challenging levels of clipping. Since the gain levels are so high, it makes the sound ‘distorted.’
Fuzz pedals clip the signal hard at the transistor stage. This makes it sound like a square wave with harmonic saturation.
Modulation pedals refer to those that take the guitar signal, identify specific characteristics of it and then continuously change them. Some of the most common types of modulation used are: