The ERD shows the main data objects for an enterprise or unit and the relationships among those data objects. The three basic -components of the ERD are Entities, Attributes and Relationships. An entity type corresponds to a table within a relational database. An entity instance corresponds to a row or record within that table. An entity within an ERD is depicted as a rectangle.
Attributes correspond to the field names or column titles within the data table.
Attributes within an ERD are depicted as ovals attached to the entity shape or in a separate list. The primary key is the attribute that uniquely identifies each instance of an entity. Sometimes more than one attribute may need to be combined to create a primary key. Primary key attribute names are generally underlined. The relationship shows how the data are related to one another within the organization or unit in question.