Fuzz is closely associated with Hendrix. It's all over his records. Also listen to The Stones 'Satisfaction', and some Cream; say 'Strange Brew', for example.
Gain's a very vague one, but if you turn up the gain, you'll get that nicely 'warmed up' tone; that's drive/overdrive. Turn it up some more, until things get a bit nastier; that's actual distortion.
The pedals you can buy conform to roughly these sounds. Overdrive is probly about the most versatile one for bass, but they're all fun in their own way!
Quote by spoonmeatloaf
Whats the diffrence bewteen fuzz, gain, overdrive, drive and distortion?

Gain is the act of boosting a signal. Drive is often used instead, but generally they mean the same thing.

Overdrive is the sound produced by a tube amp being pushed too hard. Think of the classic rock guitar sound and you're probably close.

Distortion is overdrive, but generally a lot more of it. Think death metal bands.

Fuzz is similar to od and distortion in that it is a for of signal clipping, but it tends to produce more angular waveforms, which sound "fuzzy". Hendrix is the classic example, but in bass terms, think of Cliff Burton on Anaesthesia (Pulling Teeth).
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Quote by ScottB
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From the standpoint of the physics of sound waves (note: Amplitude refers to the height of the wave (loudness))

Gain-Actively making the amplitude of the wave larger (louder)

Distortion-This happens when the amplitude of a wave gets bigger than the medium its traveling through can handle. Artificial distortion in pedals is created by taking a wave and compressing the amplitude

Fuzz-Ultra compression of the amplitude of a wave. Essentially cutting off the edge of the wave.