In Domain Driven Design, we often separate Entities and Value Objects. Junior architects always get confused between these 2 concepts.
The essential difference is that domain entities have an identity and a lifecycle. So each Entity has a unique identity and with a given domain, no two entities can have the same identity. Value objects need not have an identity. So if we have an "equals()" method that compares the parameter values of each value object, then we can have value objects that are identical. Value objects should ideally also be immutable.
The following links offer interesting stuff on this concept.