A knob is a cap-shaped piece of hardware that slips over a potentiometer or switch. Turning the knob makes adjustments to voltage, or turns equipment on an off.
Knobs and dials are generally marked with an indicator that points to a voltage value. The value can be numbered or represented by an icon or bar that gradually increases in size. Turn the knob in either direction turns the attached potentiometer, adjusting voltage accordingly. They can adjust the speed of a drive or motor, or change the volume on an amplifier.
Counting dials are specific types of knobs that are designed for precision potentiometers. The knob has a built-in counter that indicates the number of turns made. The count is used to dial into a very precise position, and the counting dial can be locked to maintain that position.