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 Definition: An entity-relationship (ER) diagram 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. 

 a one to one relationship is where each record in a table(A) can have only one matching record in table(B). This type of relationship is not common, because most information related in this way would be in one table.  Example:

 A one to many relationship is the most common type of relationship. This is where a record in a table (A) can have many matching records in table(b).  For example one suppler can supply more than one product to a company, but each product has only one supplier.

 A many to many relationship is where a record in table (A) can have many matching records in table (B), and record in table (B) can have many matching records in table (A).