Define Data Flow Diagram

DEFINING DATA FLOW DIAGRAMS (DFDs)
When it comes to conveying how information data flows through systems (and how that data is transformed in the process), data flow diagrams (DFDs) are the method of choice over technical descriptions for three principal reasons.

1. DFDs are easier to understand by technical and nontechnical audiences
2. DFDs can provide a high level system overview, complete with boundaries and connections to other systems
3. DFDs can provide a detailed representation of system components

DFDs help system designers and others during initial analysis stages visualize a current system or one that may be necessary to meet new requirements. Systems analysts prefer working with DFDs, particularly when they require a clear understanding of the boundary between existing systems and postulated systems. DFDs represent the following:

1. External devices sending and receiving data
2. Processes that change that data
3. Data flows themselves
4. Data storage locations

The hierarchical DFD typically consists of a top-level diagram (Level 0) underlain by cascading lower level diagrams (Level 1, Level 2…) that represent different parts of the system.

Reference : http://ratandon.mysite.syr.edu/cis453/notes/DFD_over_Flowcharts.pdf