E-R diagram, also known as entity relation diagram, is used to represent the relationship between entity sets. It provides methods to express entity types, attributes and connections; A conceptual model used to describe the real world. ER model is the design or blueprint of database, which can be realized as a database in the future.

In an E-R diagram, an entity set is a group of similar entities (data objects in a data model), which can have attributes. In the database system, entities are tables or attributes in the database, so ER diagram shows the complete logical structure of the database by showing the relationship between tables and their attributes.

Entities represent data objects or components in a data model. For example, people, students and music can all be regarded as data objects. It is represented by a rectangle in the electrorheological diagram.

There is a special entity in the data model: weak entity, which cannot be uniquely identified by its own attributes and depends on its relationship with other entities. In the electrorheological diagram, weak entities are represented by double rectangles.