Fundamental difference between microcontroller and PLC
I would describe the difference between PLC and microcontroller as follows.

Yes, a PLC is a microcontroller "ruggedized and adapted for the control of manufacturing processes". 

The difference is that a PLC usually has a user-friendly wrapper already built-in, which provides easy and  reliable access to various IO functions, ADCs and DACs, timers, alarms, etc. The programming interface is tailored to industrial control functions, and is not unnecessary overflexible as BASIC or C.

If I try to provide a smooth one-line formulation, PLC are microcontrollers with pre-compiled interface to standard I/O functions and control loops.