An automatic feedback control system is composed of three key components: sensors that measure the state of a system, actuators that can manipulate system variables, and a decision-making system, controller, that uses information from the sensors to decide how to modulate the actuators to achieve a desired behavior in the system.
For instance, by sensing roll, pitch, and yaw, the control system of an airplane can autonomously fly the craft in a smooth manner by controlling the actuators (namely, the fuel injection and control surface deflections) even during significant wind disturbances. The job of a control engineer is to analyze the system dynamics (e.g. the airplane flight dynamics), and consequently determine the control algorithm for the actuators that will result in the desired behavior for the system (e.g. plane flying at constant altitude and velocity despite wind disturbances).
Control systems are an essential component in aerospace systems, space exploration, autonomous robots, automotive systems, manufacturing, and in general any intelligent or autonomous system. With its emphasis on analysis and design for complex systems, students frequently find control a fascinating area of study.