Pneumatic: A system in which gas pressure differences and their rates of change are related to gas flows, their integrals and their rates of change. Hydraulic: A mechanism operated by the resistance offered or the pressure transmitted when a liquid is forced through a small opening or tube System – An interconnection of elements and devices for a desired purpose. Control System – An interconnection of components forming a system configuration that will provide a desired response. Process – The device, plant, or system under control. The input and output relationship represents the cause-and-effect relationship of the process.