An entity-relationship diagram (ERD) is a visual representation of the relationships between different entities in a database. Entities can be objects, concepts, or events that have data stored about them, such as customers, orders, or products. ERDs help in designing and understanding the structure of a database by illustrating how these entities interact with one another. In an ERD, entities are typically represented as rectangles, while the relationships between them are shown as lines connecting these rectangles. Attributes, which are the properties of entities, are often depicted as ovals. This diagramming technique is essential for database design, ensuring that all necessary data relationships are clearly defined.