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.