I would like to argue about this with Mr. Jabber.
Real time system does't only mean it is not a demonstration propram. Actually it is a special field of computer applications.
A Real-time system must not only produce logically correct results, but it must also produce them with certain timing constraints. That is, the application has to produce required signals at required time points. So this field is strongly related to real time signal processing . In fact real time systems are mostly applied in industrial controls, automated factories, telecommunications, computer speech synthesis etc, which are all timing critical applications.
By large, real time system includes the steps of controlling the real life objects. So programmers usually have to have the knowledge of automatic control theory.