Process control consists of  three instrumentation elements. Measurement, evaluation, and final control. The final control element is probably the most important because it exerts a direct influence on the process. Final control devices contain the essential pieces of equipment to convert the control signal (generated by a process controller) into the action needed to correctly control the process. 

Control valve is a final control device. A control valve is simply a variable orifice that is used to regulate the flow of a process fluid according to the requirements of the process.