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 Customer, Order, or Product. The diagram uses symbols to illustrate how these entities interact, making it easier to understand the structure of the database. ERDs typically include entities, attributes (details about the entities), and relationships (how entities are connected). This tool is essential for database design, helping developers and stakeholders communicate effectively about the data requirements and organization within a system.