What is a programmable logic controller (plc) ?

A Programmable Logic Controller, or PLC, is more or less a small computer with a built-in operating system (OS). This OS is highly specialized and optimized to handle incoming events in real time, i. e., at the time of their occurrence.

PLC's control everything from MRI machines to the automotive industry, rollercoasters etc. including spinning of turbines in Nuclear facilities. Stuxnet was developed jointly by the US and Israel deployed to specifically target the PLC's in Iran's nuclear facilities, by reporting false data back to the control room and the centrifuges for separating nuclear material would be causing the fast-spinning centrifuges to tear themselves apart