A data flow diagram is a graphical representation of the "flow" of data through an information system

Information system

An information system - or application landscape - is any combination of information technology and people's activities that support operations, management, and decision making. In a very broad sense, the term information system is frequently used to refer to the interaction between people,...

, modelling its process aspects. Often they are a preliminary step used to create an overview of the system which can later be elaborated. DFDs can also be used for the visualization

Data visualization

Data visualization is the study of the visual representation of data, meaning "information that has been abstracted in some schematic form, including attributes or variables for the units of information"....

 of data processing

Data processing

Computer data processing is any process that a computer program does to enter data and summarise, analyse or otherwise convert data into usable information. The process may be automated and run on a computer. It involves recording, analysing, sorting, summarising, calculating, disseminating and...

 (structured design).

A DFD shows what kinds of data will be input to and output from the system, where the data will come from and go to, and where the data will be stored.