Entity Relationship Diagrams (ERDs) are visual diagrams used to represent the relationships between entities in a database. ERDs are used to illustrate the structure of a database and the relationships between different entities within the database. They are used to model the data and the relationships between different entities,
From Wikipedia
An entity–relationship model describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types and specifies relationships that can exist between entities.