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. The diagram uses symbols to illustrate how these entities interact, making it easier to understand the structure of the database. ERDs typically include rectangles to represent entities, diamonds for relationships, and ovals for attributes. By mapping out these components, an ERD helps database designers and developers organize data efficiently and ensures that all necessary relationships are accounted for in the database design process.