An example about heating up of a room temperature which is a process that has the specific, desired outcome to reach and maintain a temperature (e.g. 20°C) defined by you or a person inside the room and should be kept constant over time. You can always change the setting from time to time depending on what your body wants. Get the remote control and adjust the setting to your desired temperature.
In the example above, the temperature is the process variable (PV). At the same time, it is the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat up the room as you prefer. The desired temperature (20°C) is the setpoint (SP). The state of the heater, for example the setting of the valve allowing hot water to flow through it or in the case of simple on-off control, turning the heater off and on is called the manipulated variable (MV) since it is subject to control actions. The first instant is an example of a Proportional control since the opening of the valve is proportional to the needs of the users of the room. There are other types of control such as Derivative and Integral or a combination of the two incorporated with Proportional or what we call PID. This are all common in controlling other parameters such as pressure, flow and level of a material (liquid or solid).
A commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs.
Using the example in the previous paragraph, the room temperature would be an input to the PLC. The logical statements would compare the setpoint to the input temperature and determine whether more or less heating was necessary to keep the temperature constant. A PLC output would then either open or close the hot water valve, an incremental amount, depending on whether more or less hot water was needed or just simply turn the heater off when the desired temperature is reached and turn it back on when a specified hysteresis is meet. Larger more complex systems can be controlled by a Distributed Control System (DCS) or SCADA system. Click here to know more about the general process control loop.
Also Read: Understanding a Process Control Loop