An association represents a structural relationship that connects two classifiers. Like attributes, associations record the properties of classifiers. For example, in relationships between classes, you can use associations to show the design decisions that you made about classes in your application that contain data, and to show which of those classes need to share data. You can use an association's navigability feature to show how an object of one class gains access to an object of another class or, in a reflexive association, to an object of the same class.
The name of an association describes the nature of the relationship between two classifiers and should be a verb or phrase.
In the diagram editor, an association appears as a solid line between two classifiers.
An association end specifies the role that the object at one end of a relationship performs. Each end of a relationship has properties that specify the role of the association end, its multiplicity, visibility, navigability, and constraints.
In an e-commerce application, a customer class has a single association with an account class. The association shows that a customer instance owns one or more instances of the account class. If you have an account, you can locate the customer that owns the account. Given a particular customer, you can navigate to each of the customer’s accounts. The association between the customer class and the account class is important because it shows the structure between the two classifiers.