SCADA stands for Supervisory Control and Data Acquisition .
As the name states, it does two things: supervisory control and data acquisition .
SCADA is basically a control system spread geographically over multiple sites.
A good practical example where this is used is by electric utility companies where they can control multiple unmanned substations remotely from a central control center .
How does SCADA do this ?
Each substation would have a RTU (Remote Terminal Unit) which is basically a Controller (PLC) to which all the local sensors and equipment wire to . In a substation think of this where the breakers, disconnect switches etc would be connected so you could get status , remotely control them.
The Substation RTU needs to be connected to server (main computer) in utility control center. This is the communications physical medium which could be radio, microwave, leased telephone line) . Communication is done through various communication protocols (languages).
At utility control center , you have the server (main computer) and operator workstations (given fancy name Human Machine Interface) whereby the human operator can interact with the SCADA software and see the various alarms , change the setpoint etc).
This is basically supervisory control whereby this information gets sent to the various RTU’s in the substation .