The big difference is that a microcontroller and its software/firmware tools are intended for general purpose electronic applications. Typically you buy a development kit, code and debug your source code on that. The actual product that you are going to sell has the microcontroller chip embedded in it, and at some stage in the production process the compiled code from the development kit is burned into the target micros.
The end product could be anything from any piece of consumer electronics, to some sophisticated gizmo buried in a piece of military hardware, but usually characterised by:
1. A relatively low I/O count.
2. No end user programmability, scalability or expansion.
3. Relaxed enviromental specs.
4. Short product lifetimes and spares availability, and zip to none aftersales support.
At the lowest sizes a small PLC and a Microcontroller are somewhat similar inside, but at every other point they differ. At the larger sizes, PLC's such as Siemens S7, Schneider Quantums and Rockwell's Controllogix systems are far more complex than any microcontroller.
Most especially the degree of software and firmware development in a full-scale PLC hugely outstrips what can be achieved with any small microcontroller, indeed the effort involved would not be so very much less than that involved in producing and maintaining something like the LinuxOS, or even WIndows XP for example. The biggest difference is that PLC firmware and hardware is designed and tested to be many orders of magnitude more reliable than any commercial OS such as Windows.
And from an end-user's point of view a PLC is characterised by:
1. Can be scaled and extended to many thousands of I/O and complex industrial devices.
2. The end-user had total programmability and application control.
3. Much tighter enviromental specs and packaging ruggedness.
3. Product life-cycles of 20 years plus are the standard in Industrial Automation, with spares and support generally assured in that period.
Is that enough for your homework?