[Don Reid]

Potentiometers are what are used for volume controls etc. on sound equipment.
You can get them at places like Radio Shack even (but be sure you get a linear one between 10k and 100k).

Potentiometers are good for measuring the position of a moving part of your robot like an arm. You hook them to one of the analog inputs as shown in the RC manual from Innovation First. Then the value from the input tells how far it has been rotated.