A Value Object is an object that has no unique identity, but
rather represents a characteristic of something else. For instance, the
integer 7 can be used anywhere and always means the same thing - we don't think
about this or that particular instance of the integer 7. Likewise, a given
date like 11 July 2011 (GMT) is the same everywhere - it has no additional
identity. In designing systems, it's useful to identify parts of the design
that should be represented as Value Objects, as opposed to Entities, which each
have a unique identity.