An ER Diagram (Entity-Relationship Diagram) is a visual representation used in database design to illustrate the relationships between different entities. Entities can be anything relevant to the system, such as customers, orders, or products. Each entity is represented by a rectangle, while the relationships between them are shown with lines connecting these rectangles. This helps in understanding how data interacts within a system. In an ER Diagram, attributes of each entity are often listed inside ovals connected to their respective rectangles. For example, a customer entity might have attributes like name, email, and phone number. By using ER Diagrams, developers can effectively plan and organize data structures, ensuring a well-structured database.