Hi,
PLC stands for programmable logic controller and had in old days its own simple PLC-language to control industrial peripheral. It is designed to be very flexible and has modular I/O modules which can be configured and expanded. Thus the I/O specification is around 24V and standardized. The typical tasks of a PLC will be automating a production process. As already said an embedded system is the core of a PLC. Usually a closed system(like the mentioned coffee machine) is not modular expandable. As "embedded microcontroller" indicates it inside the system with a fixed function. If you design with a PLC an automated process and is then a complete system, the PLC becomes "embedded".
Enjoy your design work!