Entity-Relationship Diagrams (ERD) are visual representations used to illustrate the relationships between different entities in a database. An entity can be anything that holds data, such as customers, products, or orders. 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 or lines connecting the entities. Attributes, which provide more details about each entity, are often depicted as ovals. This diagrammatic approach simplifies the process of database design and enhances communication among stakeholders.