PLC

PLC is an acronym for "Programmable Logic Controller".... a fancy way to identify an industrialized computer.

Like a computer, the PLC can be "programmed" with commands to perform specific functions. Also like computers, the amount of memory and the class of CPU (Central Processing Unit) helps to determine how expansive a program the PLC can run. Some of the older and less expensive PLC's were rated in "lines of code" that they could run. A line of code was limited to a specific number of characters and the CPU could only handle some reasonable number of LINES of processing code. Times have changed and with the advent of inexpensive memory and increasing processor speeds, the PLC can perform a multitude of very sophisticated tasks for much less than it would take with other types of devices and systems.

Also like the computer, the PLC has I/O devices. The computer has "serial" and "parallel" ports, and a most recent addition, the USB port, to control peripheral devices such as printers, scanners, cameras, monitors, etc. etc.  The PLC has I/O devices too. Many of the OUTPUTS are like little switches that turn a motor starter off and on. Some are actually small relays, others are power transistors, and still others are digital or analog outputs that can be varied up and down by the PLC's program. On the INPUT side, they act like little "information takers". A push button might be connected to an input and when it is activated, the input is checked by the program and it's condition (opened or closed) is transmitted to the CPU to be used within the program instructions. These inputs can also be digital or analog signals. In this case, the inputs could possibly tell the program exactly what the temperature is at a specific time. Or it might tell the progam how many revolutions a shaft has made since the last time it was checked.

So you see, a PLC is in fact a computer that....

Collects Data
Sends the data to a processing program
The program interprets the information
And tells the output devices what they should do.