The function of any information system can be expressed in terms of transformation (processing) of certain inputs (which are data) into outputs (which are data too) where memory (which too consists of data) may need to be consulted (or updated). This suggests that two crucial elements in a system's description are data and processing of data. A complete description of an information system demands description of both these elements. In fact, we can reduce this, in a mundane fashion to the equation:
System = Data + Processing of data
While it is impossible to describe an information system exclusively in terms of data or its processing, it is possible to study a system's functions (what the system must do) in terms of the transformations it must perform of the data which is modeled separately. A coherent description of the information system, however, would require that the models of data and its processing are not inconsistent. An information system's functions, which describe its processing aspects, is modeled in the structured systems approach, as dataflow diagrams.Such a model of an information system is, for obvious reasons, referred to as functional model or process model.
A dataflow diagram consists of external entities (represented by rectangles), processes (represented by either rounded rectangles or circles), data stores (represented by either an open rectangle or two parallel lines) and dataflows (represented by arrows).