A controller is used to control (makes sense!) some process or aspect of the environment. A typical microcontroller application is the monitoring of my house. As the temperature rises, the controller causes the windows to open. If the temperature goes above a certain threshold, the air conditioner is activated.

Microcontroller and range of PIC Microcontrollers

Microcontrollers can be found in any products these days. For example modern washing machine in our house that consist of timer, button and LED contains a microcontroller. All modern cars contain microcontroller.