 An entity-relationship diagram (ERD) is a specialized graphic that illustrates the interrelationships between entities in a database.

 ER diagrams often use symbols to represent three different types of information, boxes are commonly used to represent entities, diamonds are normally used to represent relationships and ovals are used to represent attributes.

 Entity- An entity is a person, place, thing or event for which data is collected and maintained.  Relationship- A relationship captures how two or more entities are related to one another, this can be thought of as verbs, linking two or more nouns.  Attributes- An attribute is a property or descriptor of an entity.

 One entity (A) is associated with one other entity (B).  For example, in a database of employees, each employee name (A) is associated with only one ID number (B).

 One entity (A) is associated with one or many other entities (B), but for one entity (B) there can only be a one to one relationship with (A).  For example, for a company with all employees working in one building, the building name (A) is associated with many different employees (B), but those employees all work in the same building (A).

 One entity (A) can be associated with one or many other entities (B), and vice versa.  For example, for a company in which all of its employees work on multiple projects, each employee (A) is associated with many projects (B), and at the same time, each project (B) has multiple employees (A) associated with it.