This diagram shows the components of a feedback control system. A feedback control system is one where a measurement of the output of the process is taken, passed to a controller which figures out what do do, and then a signal from the controller is passed through the actuator into a process input. A continuous information flow loop exists through the process, measurement, controller and actuator blocks and it is called a feedback loop.
Click on the names in the diagram to find out more about the particular components.