What is Embedded System?  

An embedded system is a special-purpose system in which the computer is completely encapsulated by or didicated to the device or system it controls. Unlike a general-purpose computer, such as a personal computer, an embedded system performs one or a few pre-defined tasks, usually with very specific requiremetns. Since the system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product. Embedded systems are often mass-produced, benefiting from economies of scale.
Personal digital assistants (PDAs) or handheld computers are generally considered embedded devices because of the nature of their hardware design, even through they are more expandable in software tems. This like of difinition continures to blur as devices expand.
Physically, embedded systems range from portable devices such as digital watches and MP3 players, to large stationsy installations like traffic lights, factory controllers, or the systems controlling nuclear power plants.
In terms of complexity embedded systems can range from very simpel with a single microcontroller chip, to very complex with multple units, peripherals and networks mounted inside a large chassis or enclosure.