A specialized system that is part of a larger system. They are boards that are incorporated or added onto other devices. An embedded system is typically, but not exclusively, a single microprocessor/microcontroller board with the programs stored in internal (and/or external) ROM (Read Only Memory). Some embedded systems include an operating system, but many are so specialized that the entire logic can be implemented as a single program.
Some of the gadgets that have embedded computers are phones, digital cameras, and music players. Virtually all appliances that have a digital interface, e.g. cars, measuring instruments, factory automation, traffic control systems and so on. Embedded computers are everywhere!
Embedded computers are devices that perform a specific function in a higher assembly. The limitations of any particular embedded computer are the specifications for which it was designed, a specific function that it performed continuously. They have all the necessary functions to get their assigned purpose accomplished on a single PCB (Printed Circuit Board).
Embedded computers run a variety of software, including several versions of (GNU/)Linux. In addition, there are several known companies that support this market as well as companies that specialize in creating custom software to control embedded platforms for specific requirements.