Data flow diagram (DFD) is a graphical notation used to describe the data flow between various processes of a system. It is a visual representation of how data moves within a system, showing the inputs, outputs, processes, and data stores involved. DFDs are commonly used in system analysis and design to understand and document the flow of information in a system.

Data Flow Diagram:
A data flow diagram consists of four main components:
1. Processes: Processes represent the activities or functions that manipulate the data. These can be manual or automated actions that transform the inputs into outputs.
2. Data Flows: Data flows represent the movement of data between processes, data stores, and external entities. They show the flow of information from one part of the system to another.
3. Data Stores: Data stores are repositories where data is stored. These can be physical or virtual storage locations like databases, files, or memory.
4. External Entities: External entities are external systems, users, or sources/sinks of data that interact with the system. They are represented as rectangles and show the inputs and outputs exchanged between the system and external entities.

Advantages of Data Flow Diagrams:
- Clear Communication: DFDs provide a visual and intuitive representation of the system's data flow, making it easier for stakeholders to understand and communicate system requirements.
- System Analysis: DFDs help in analyzing the system by identifying the processes, data flows, and data stores involved. This allows for a better understanding of the system's functioning and potential improvements.
- System Design: DFDs assist in designing the system by providing a blueprint of the data flow. They help in determining the inputs, outputs, and transformations required for each process.
- Scalability: DFDs can be scaled or decomposed into lower-level diagrams, allowing for a more detailed analysis of specific processes or subsystems.
- Documentation: DFDs serve as documentation that can be used for future reference, system maintenance, and system modifications.

Conclusion:
Data flow diagrams are an effective graphical notation used to describe the data flow between various processes of a system. They provide a clear and visual representation of how data moves within a system, facilitating communication, analysis, and design of the system.