They do this by fully or partially opening or closing in response to input signals from controllers, which compare a desired setpoint to the actual process variable. They are also called modulating or throttling valves.
The valves use a device called a "positioner" to open and close via an electrical, hydraulic or pneumatic control signal. Positioners control the amount and direction of valve stem movement thru the actuator. The operator or actuator of the control valve can also be driven by electrical, hydraulic or pneumatic means.
The most common types of control valves are sliding stem globe and 90 degree rotary valves. They offer rugged construction with many options making them desirable for a variety of process control and severe service applications.