Introdution
Queueing systems represent an example of a much broader class of interesting dynamic systems, which we will denote as systems of flows. Few examples of queueing systems are:

1. Waiting in traffic light.
2. Sequencing of different jobs in different machines.
3. Withdrawing cash at the teller counter.
4. Buying a movie ticket at the cinema hall ticket counter.
5. Holding the telephone as it rings.
6. Flow of computer programmes through a time sharing computer system.

In a flow system we say that some commodity is flowing or is moving or is being transferred through a finite capacity channel in order that it goes from one point to another point. Generally flows are
(i) steady flow and
(ii) unsteady flow.