Represented as a line connecting two entities or entity sets. These are descriptive attributes of the relationship. Entities take on roles in relationship. Roles are represented by labels on the lines between relationships and entities. Degree of a relationship identifies the number of entities.
Binary relationships are most common.
Higher-degree relationships can be modeled as a set of binary relationships. Seems obvious, but isn't always. The logical structure of the data often imposes some constraints. Existence Entity X is existent dependent on Entity Y if X can only exist if Y exists. X is subordinate entity.