Relationship type vs. relationship set

Relationship Type:
Is the schema description of a relationship. Identifies the relationship name and the participating entity types. Also identifies certain relationship constraints.

Relationship Set:
The current set of relationship instances represented in the database. The current state of a relationship type. Previous figures displayed the relationship sets Each instance in the set relates individual participating entities – one from each participating entity type.

In ER diagrams, we represent the relationship type as follows:
Diamond-shaped box is used to display a relationship type.

Connected to the participating entity types via straight lines.