A Programmable Logic Controller (PLC) is a specific kind of digital computer that is commonly used in manufacturing to control specific electro-mechanical processes. The need for PLC devices came out of the automotive sector in the United States in the late 1960s as new models and parts were being developed, and it became increasingly obvious that hard-wired control panels were cumbersome and expensive to alter as industry needs were changing.
In a very real sense the development of PLC devices came about because of the need for manufacturing industry to speed up its change processes, and be better able to re-tool its plants in light of changing industry and sales conditions.
The way a PLC works is that in the same way you can program a piece of software to perform a task over and over again. A PLC can be used to perform an industrial process such as controlling the way a car part is moved around a factory, how high a temperature a part is exposed to, or the way that the machinery should operate if some particular pattern of operations occurs.
The major difference between a PLC and your home computer is in the operating conditions that the units are designed to operate.
Whereas your home Windows Operating system might crash several times a day, and it really makes no difference, apart from a few minutes inconvenience each time to reboot. With industrial machinery, any downtime can cause major headaches for the owner of the factory both in terms of lost production, and extra expense to pay workers for time they are not working. Additionally, because factories are now more automated, there are often fewer people available to fix problems if they do arise.
Hence, PLCs are designed to be highly reliable, designed to be used for long-periods without maintenance, but at the same time able to adapt to changing manufacturing processes by using complex logic systems. This means that they can adapt to variations in the factory manufacturing process better than the software we use at home on our computers; which gives the factory owners both flexibility in the way that parts are produced and increased uptime in production levels at their plants.