The document discusses HVAC (heating, ventilation, and air conditioning) systems, including their key components and control methods. It describes the components of a typical HVAC system such as chillers, cooling towers, boilers, air handling units, VAV boxes, and ductwork. It also discusses the basic control concept of using sensors, controllers, and actuators to regulate temperature, humidity, airflow and other factors. Finally, the document provides an example diagram of an air handling unit control system showing common sensors, the building management system, and actuated devices.