What is a micro controller? Where they are used?

A highly integrated chip that contains all the components comprising acontroller. Typically this includes a CPU, RAM, some form of ROM, I/O ports, and timers. Unlike a general-purpose computer, which also includes all of these components, a microcontroller is designed for a very specific task -- to control a particular system. As a result, the parts can be simplified and reduced, which cuts down on production costs.
Microcontrollers are sometimes called embedded microcontrollers, which just means that they are part of an embedded system -- that is, one part of a larger device or system.

A micro controller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals.

A microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used as an embedded system. The majority of microcontrollers in use today are embedded in other machinery, such as automobiles, telephones, appliances, and peripherals for computer systems.