Wah-wah (or just "wah")
This is effectively a foot-operated parametric equaliser.
A frequency is determined normally by the position of a foot pedal. The input signal is boosted at, and around, this frequency, above the rest of the signal. As you move the pedal, the frequency being amplified changes up or down. The frequency range is set so that it sits well with a guitar, which in turn isn't hugely dissimilar to the human voice's frequency range.
The end result is that at the lower end of the the wah-wah's frequency you get a "wwwww" kind of effect and it transforms at the higher end to a "aaaaah" sound, hence the name.
The effect of controlling the relative volumes of partials of a tone is regularly done by humans during speech, creating the distinct tones we call vowel sounds. The "wah" sound we perceive is a digital analogue of this human speech pattern, and our brains hear the variations in the tone as a speech pattern of "wwwwwwahhhh". (The "w" here is more like a "u" or "oo" sound, hence vowel sounds only)
The pedal rocks forward and backward like a see-saw as you rest your foot on it. Move the pedal to get the wah effect. On some pedals, there is a switch under the toe end to switch ceon straight-through (no effect) to using the wah effect. This means that when you switch it on, it's always in the "aaaa" position. You can hear this in some of Hendrix's work.
Others just detect that the pedal has moved, and switch the effect on. When you stop moving the pedal, it switches off.
On some wah pedals, the signal-with-effect ends up slightly louder than the original signal because a portion of it is being boosted. Other (normally more modern) ones cater for this by bringing the overall signal down a little when using the effect, so that you just get the effect and not extra output.
To hear a classic example of this effect, listen to the beginning of Hendrix's Voodoo Child (slight return)
