Like distortion and overdrive, fuzz pedals use a resistor or other analog component to clip the incoming signal. The Maestro Fuzz Tone is widely recognized as the first mass-produced bucket-brigade pedal.
Unlike OD and other drives, however, fuzz usually has no reverence for your tone. Scrambled, seering, velcro, sputtery, and destroyed are all adjectives that can describe fuzz tones.
Many fuzz pedals were originally designed to make guitars sound like other instruments, like horns or cellos. Some unique fuzz pedals may add additional effects, such as gate, octave up/down, or filters. The Zvex Fuzz Factory adds oscillation for even noisier possibilities.