Inheritance expresses what relationship between the derived class and the base class?

Inheritance and "Is a" Relationships
Two ways to show class relationships in object-oriented programming are "is a" and "has a" relationships. In an "is a" relationship, the derived class is clearly a kind of the base class. For example, a class named PremierCustomer represents an "is a" relationship with a base class named Customer because a premier customer is a customer. However, a class named CustomerReferral represents a "has a" relationship with the Customer class because a customer referral has a customer, but a customer referral is not a kind of customer.