There are three main types of relationships depicted in an ER diagram:
One-to-one:A one-to-one relationship between two entities means that each entity in the first entity can be related to at most one entity in the second entity, and vice versa. For example, a person can have at most one passport, and a passport can be issued to at most one person.
One-to-many:A one-to-many relationship between two entities means that each entity in the first entity can be related to zero or more entities in the second entity, but each entity in the second entity can be related to only one entity in the first entity. For example, a customer can place zero or more orders, but each order can be placed by only one customer.
Many-to-many:A many-to-many relationship between two entities means that each entity in the first entity can be related to zero or more entities in the second entity, and vice versa. For example, a product can be sold in zero or more stores, and each store can sell zero or more products.