SCADA stands for supervisory control and data acquisition. It is the control-room software through which operators watch the whole plant on screens and steer it. Supervisory means that it watches over the plant and sends commands, but the controllers beside the machines do the fast, second-by-second control.
SCADA collects readings from many PLCs and from the distributed control system, and shows them on diagrams of the plant. It shows an alarm when a value goes out of range. It lets an operator send a command, such as starting a pump or changing a set point. It also sends every reading to the process historian to be stored.
Say you work as an operator at a dairy plant in Anand. The milk must be held at 72 °C for 15 seconds so that harmful germs are killed. This is called pasteurisation. On your screen the milk temperature shows 72.4 °C, in green. Suddenly it falls to 70.8 °C, and the number turns red with an alarm sound. You click on the diversion valve on the diagram. The milk that was not hot enough goes back for reheating, and your command reaches the PLC in less than a second.
SCADA is used outside factories as well. An electricity grid uses a version called an energy management system. An oil field uses SCADA data in production accounting, which works out how much oil and gas each well produced.
SCADA also reaches places that are far from the control room. A water board that runs pumping stations all over a city, or a gas pipeline that is hundreds of kilometres long, cannot keep an operator beside every pump. Each remote site has a small controller called a remote terminal unit (RTU). The RTU sends its readings to the central SCADA over radio, a mobile network or a satellite link, and carries out the commands that come back. If the link is broken, the RTU keeps the site running on its own.
Higher in the factory, SCADA reports production counts to the manufacturing execution system. Because it can send commands to machines, it is a prime target in an attack, so it is protected by OT and IoT security.