A PLC is a special microcontroller designed for industrial use, that is for controlling machinery or processes. Usually a PLC is programmed using Ladder diagrams and specialized control software.
A microcontroller is a microprocessor that can be used for any type of application, but there some for special applications, like a PLC. Here's a link.
http://en.wikipedia.org/wiki/Microcontrollerhttp://en.wikipedia.org/wiki/Programmable_logic_controller
