An Entity-Relationship Diagram (ERD) is a tool used in database design to model the structure of a database by depicting entities, their attributes, and the relationships between them. Entities represent objects or concepts within the system, such as customers or products, while attributes describe properties of these entities, like customer names or product prices. Relationships illustrate how entities interact with each other, such as one-to-many or many-to-many associations. ERDs help in visualizing the database schema, designing data structures, and understanding data relationships, making them essential for effective database planning and management.