Although the distinction is very subtle, I think the answer can be most easily found in the names themselves:

microPROCESSORs and microCONTROLLERs
-uPs are more optimized towards Processing (of data)

-uCs are optimized more for CONTROLLING (of motors, processes etc.)

Still the more specific differences are covered in Erik's list, and here:
* uPs have internal timers, more/better I/Os, bit-operations, even ADCs/DACs (as in PICs)
* uCs have better/more data handling (eg addressing modes, indices) and processing (eg comparison ops, arithmetic ops) features

Hopefully this posting addressed the crux of the issue :)

Kundi