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An ER diagram (ERD), also called an entity relationship model, is a graphical representation of a system's entities and their relationships to each other.
Entities refer to people, things and actions, etc., while relationships refer to the inter-relations of the entities. For example, when the event in which "an employee places an order for a product" is applied to an ERD, the "employee" and the "product" represent the entities and "places an order" which connects these entities represents the relationship.
ERDs are mainly used for the purpose of arranging the data structure when designing a relational database.
The notation methods for ERDs include the Peter Chen notation, advocated by Dr. Peter Pin-Shan Chen who developed the ER model, IDEF1X and IE.