The Entity Relationship Diagram is a graphical representation of the relationships between entities in a system. It is used to model the data and relationships between entities in a database. The diagram is composed of entities, which are represented by rectangles, and relationships, which are represented by lines connecting the 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.