Microcontrollers (often shortened to MCUs or MCs) are extremely small microcomputers that are entirely self-contained on a single chip.
You can define a microcontroller as a simplified computer - one that is generally designed to run a single basic programme repeatedly. By definition, microcontrollers are typically intended to perform a single automated task, as pre-programmed by the user, in a single device. They are designed to do this one job repeatedly (or, as is also common, on a timed loop).
This is known as an embedded application, as opposed to the more versatile, general-purpose applications handled by full microprocessors and CPUs.
Microcontrollers do contain a microprocessor of sorts as one of their key components, but it is typically a far less complex and dynamic form of CPU than most standalone MPs. This is because the microcontroller unit is generally limited to performing a single highly specific job. This means that it does not need the full range of functionality that a proper microprocessor offers.
To achieve this, the basics of a microcontroller dictate that it generally works in conjunction with other types of components and electronic circuits, connected via printed circuit boards (PCBs). This combination of a microcontroller and PCB-based equipment can be used to play a key role in controlling, monitoring and affecting various sorts of systems and component behaviours.