What is a PLC?  programmable logic controller
PLCs are ruggedized microcomputers with hardware and software designed to perform industrial control operations by example: automated packing, filling, robots, lighting, smart buildings, and applications only limited to the imagination.
a PLC consists of two basic sections:

• The CPU (central processing unit) controls all PLC activity into processor and memory system
   

• The input/output interface system is physically connected to field devices such as switches, sensors, lights, and motors, and provides the interface between the CPU and the field devices (inputs and outputs)

PLCs are used to control machines or processes that are sequential or continuous that use analog and digital I/O.