Gain effects will pass the guitar signal through a transistor or diode to produce the clipping sound of a tube amplifier cranked up loud. Most players call these effects distortion pedals, but there are different varieties of distortion that produce distinctly unique tones, all driven by the amount of gain you push. There are 3 types of gain effects
Overdrive - the sound of a tube amp cranked up to high volumes. Tube screamer circuit is one of the most popular of OD or "crunch" sound
Fuzz - Fuzz tone basically is a heavily saturated guitar signal, altering the sound to a plain square wave with amazing sustain. What does fuzz sound like? Listen to Jimmy Hendrix or Black Sabbath guitar. Some modern fuzz add an octave effect.
Distortion - this is the guitar effect used by most heavy metal genre guitar players. It is the "buzzsaw" sound. Greater harmonic saturation, more audible overtones, and more sustain than a clean signal. There are many different varieties of this most used gain effect.