Value objects

An object that represent a descriptive aspect of the domain with no conceptual identity is called a VALUE OBJECT. VALUE OBJECTS are instantiated to represent elements of the design that we care about only for what they are, not who or which they are.

When you care only about the attributes of an element of the model, classify it as a VALUE OBJECT. Make it express the meaning of the attributes it conveys and give it related functionality. Treat the VALUE OBJECT as immutable. Dont' give it any identity and avoid the design complexities necessary to maintain ENTITIES.

Eric Evans, "A Model Expressed in Software", in Domain-Driven Design: Tackling Complexity in the Heart of Software, 98-99.