What is cascade control and when is it used?

Cascade control uses two controllers in series - a primary (master) controller sets the setpoint for a secondary (slave) controller. The secondary loop controls a faster, inner variable that affects the primary variable. Example: temperature control of a reactor where primary controller outputs setpoint to steam flow controller. Cascade control improves response to disturbances in the secondary loop before they affect the primary variable, providing faster correction and better control quality.