Control valve actuators control fluid in a pipe by varying the orifice size through which the fluid flows. Control valves contain two major components: the valve body and the valve actuator. The valve body provides the fluid connections and a movable restrictor comprised of a valve stem and plug that is in contact with the fluid that varies the flow. The valve actuator is the component that physically moves the restrictor to vary the fluid flow.
The video below introduces the viewer to basic control valve operation concepts and the basic types of actuators.