A class hierarchy or inheritance tree in computer science is a classification of object types, denoting objects as the instantiations of classes (class is like a blueprint, the object is what is built from that blueprint) inter-relating the various classes by relationships such as "inherits", "extends", "is an abstraction of", "an interface definition". In object-oriented programming, a class is a template that defines the state and behavior common to objects of a certain kind. A class can be defined in terms of other classes.