sentences:  It is common practice to draw a context-level data-flow diagram first which shows the interaction between the system and outside entities.The DFD is designed to show how a system is divided into smaller portions and to highlight the flow of data between those parts.This context-level data-flow diagram is then "exploded" to show more detail of the system being modeled
  
  An Information model is not a type of data model, but more or less an alternative model.