Data flow diagrams (DFDs) are a powerful tool used to visualize the flow of data within a system. They are used to identify and document the flow of data from its source to its destination. A DFD can help you understand how data is processed
From Wikipedia
A data-flow diagram is a way of representing a flow of data through a process or a system. The DFD also provides information about the outputs and inputs of each entity and the process itself. A data-flow diagram has no control flow — there are no decision rules and no loops. Specific operations based on the data can be represented by a flowchart.