Ever wondered what is inside your guitars tone or volume knob? This article at geofex.com is a great introduction to understanding how your guitar controls work. The controls are potentiometers (pots for short) : a type of variable resistor.

Turning the knob moves a wiper around a conductive strip of material, changing the resistance of the potentiometer as it goes. Eventually the wiper can wear a path through the conductive strip, leading to crackly noises. If all the contacts get all the way through the strip the potentiometer will go completely dead. The article suggests that some brands of pot can be repaired using various tricks.

Link: geofex.com sectrets of potentiometers
Via: makezine

The Tone Files has a useful online form for finding out the date of your potentiometer (and by inference your guitar, amp or effects pedal). Look on the back of your potentiometer. There will be a six or seven digit number stamped into the metal (there may be a hypen a space or a period as well). Enter your pot code at this link .