This fourth edition of Programmable Logic Controllers continues to provide an up-to-date introduction to all aspects of PLC programming, installation, and maintaining procedures. No previous knowledge of PLC systems or programming is assumed. As one reviewer of this edition put it "I honestly believe that someone with little or no background to PLC systems could take this book and teach themselves PLCs"
What is a PLC (Programmable Logic Controller)?
A Programmable Logic Controller, or PLC, is more or less a tiny computer with a built-in operating system (OS).
This OS is highly specialized to handle incoming events in real time, i.e. at the time of their occurrence. The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and it has output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.) The system is user programmable. It uses a language called "Relay Ladder" or RLL (Relay Ladder Logic). The name of this language implies the fact that the control logic of the earlier days, which was built from relays, is being simulated.
 The PLCs purpose in life
The PLC is primarily used to control machinery. A program is written for the PLC which turns on and off outputs based on input conditions and the internal program. In this aspect, a PLC is similar to a computer. However, a PLC is designed to be programmed once, and run repeatedly as needed. In fact, a crafty programmer could use a PLC to control not only simple devices such as a garage door opener, but their whole house, including turning lights on and off at certain times, monitoring a custom built security system, etc.
Most commonly, a PLC is found inside of a machine in an industrial environment. A PLC can run an automatic machine for years with little human intervention. They are designed to withstand most harsh environments a PLC will encoun.