An association is a class that represents a relationship between two or more classes. Associations enable the creation of multiple relationship instances for a given class. System components can be related in many different ways, and associations provide a way of representing the relationships of these components.
Because of the way associations are defined, it is possible to establish a relationship between classes without affecting any of the related classes. The addition of an association does not affect the interface of the related classes. Only associations can have references.