What does an 'Entity Relationship Diagram' illustrate?

An Entity Relationship Diagram (ERD) is specifically designed to illustrate the entities within a system and the relationships between those entities. This type of diagram is a vital tool in database design and serves to visually represent the structure of a database by mapping out the various entities—such as tables in a relational database—and how they interact or connect with one another.

For instance, in an ERD, entities may include objects like "Customer," "Order," and "Product," while the relationships demonstrate how these entities function together, such as a customer placing an order or an order containing multiple products. By focusing on the connections between entities, ERDs help in understanding the data requirements and can guide the development of a well-structured database schema.

Other options do not accurately describe the purpose of an ERD. Representations of physical devices pertain more to network architecture. Flowcharts typically illustrate processes or workflows rather than relationships between data entities. An organizational hierarchy focuses on the structure of an organization rather than how data entities relate to each other. Thus, the correct understanding of an Entity Relationship Diagram lies in its focus on showcasing entities and their interrelations.