A microprocessor is usually understood to be a single-chip central processing unit (CPU), with the CPU being the "brains" of a computer. A microcomputer is any computer built around a microprocessor, along with program and data memory, and I/O devices and other peripherals as needed. A microcontroller (often shortened to “μC” in this tutorial) is a single chip device which has on board not only a microprocessor but also, on the same chip, nonvolatile program (ROM) and volatile data (RAM) memory, along with useful peripherals such as general-purpose I/O (GPIO), timers and serial communications channels. Thus it follows that all microcontrollers are microcomputers, but not all microcomputers use microcontrollers.
In smaller embedded systems it is most common to use microcontrollers since they give the most compact design and the lowest hardware cost. Larger embedded systems, on the other hand, may use one or more microprocessors if a microcontroller of suitable speed and functionality cannot be found. It is also possible to include both microprocessors and microcontrollers in a complex embedded system. The only rules are, use whatever device(s) fit the task, given the constraints on budget, availability, time, tools, etc.
It should also be pointed out that with most microcontrollers it is possible to add external memory and peripherals, should the on-board mix not take care of all the system needs. When it makes sense to add such external devices, as opposed to choosing a larger microcontroller with the needed resources on-board, is a choice that needs to be made on an individual design basis.
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