An Entity Relationship (ER) diagram is a visual representation of entities and their relationships to each other, used in database design. It uses symbols and shapes to represent entities, attributes, and relationships between entities. ER diagrams are used as:

1. Data modeling: To model and represent the data and relationships between entities in a database system.

2. Communication tool: To clearly communicate the structure of a database system and the relationships between entities to stakeholders.

3. Database design: To design and plan the structure of a database, including entities, attributes, and relationships.

4. Requirements gathering: To gather and analyze business requirements and translate them into a database design.

5. Database documentation: To document the structure of a database and provide a clear and comprehensive understanding of the relationships between entities.

ER diagrams are a useful tool in the analysis of relational data and can help establish relationships between tables in a database system.