| There's guitarnuts.com, that has some good stuff.
Basically, in a passive circuit (no preamp, just vol&tone), you can't add anything to the signal - only take away. So when volume and tone are at full, there's zero cut being applied.
In a volume pot (potentiometer), as you turn down the volume you're sending an increasing amount out to ground. As the pot turns down from full, internally there's a wiper which makes the path to ground shorter.
Similarly in a tone pot, as you turn down from full more of the signal goes to ground via the capacitor. Different capacitors affect what frequency ranges go to ground. A tone control is different from an active treble boost/cut as the active contol only boosts/cuts it's frequency range, whereas a tone control rolls off all frequencies past the range implied by the capacitor. | 
