Entity-Relationship Diagrams (ERDs) are visual representations used to illustrate the relationships between different entities in a database. An entity can be anything that holds data, such as a customer, product, or order. ERDs help in designing and understanding the structure of a database by showing how entities interact with one another. In an ERD, entities are typically represented as rectangles, while relationships are shown as diamonds connecting these rectangles. Attributes, which provide more information about entities, are depicted as ovals. This clear layout makes it easier for database designers and developers to communicate and plan the database structure effectively.