This document discusses control valves, including their classification, basic parts, types, and how they work. Control valves can regulate fluid flow and control process variables under controller commands. There are two main types: linear and rotary. The basic parts include a valve body, bonnet, plug, trim, and actuator. Actuators are either pneumatic or electric and convert control signals into valve stem movement. Positioners translate control signals into standardized signals for the actuator. Trim parts like the stem, seat and plug are exposed to the process flow.