overdrive = it is when the input gain is cranked to the point of the output becoming so hot, the sound distorts. good example, black sabbath records.
distortion = the signal in the chain is manipulated to the point that the sonic lines become jagged, giving you a raspy, distorted sound. example, metal bands or basically anything you hear today thats promoted as "rock."
fuzz = just like distortion except the sound waves are manipulated to become square, giving you a buzzy, very distorted sound. example, hendrix and smashing pumpkins' earlier albums (when corgan still had hair)