As the word implies it meas the system is generally "embedded" into a larger system that could very well be mechtronic or electromechanical such as a missle or could be purely electrical such as a handheld personal diary. In a certain way it is an intelligent electronic system that could comprise of mirco/controller/processor, FPGA, or other decision making device. An embedded system has certain aspects such as it usually has real time processing, it is normally small and uses few available resourses for memory and storage, it is often fast and it is a specialized computer doing specific stuff i.e., it cannot run everyday computer software although that is changing now with the newer PDA and cell phones that have common software for both PC and embedded devices. Embedded systems used to be stand alone in past but then again that has changed with devices for example that run Windows CE that have a user interface.
That's about it.