Consider the following "is-a" or inheritance relationships between "concrete" entities Coca Cola, sulfuric acid, milk and the "abstract" liquid (named "ALiquid" by convention since it is an abstract entity):
The UML diagram shows us that Coca Cola, sulfuric acid and milk are all liquids. Conversely, the diagram tells us that a liquid could be either Coca Cola, sulfuric acid or milk. Note of course, that liquids are not constrained to these 3 entities but that doesn't affect the discussion here—in fact, this will be an important feature later on.
Another way that we can express the notions depicted by the diagram is to say that the abstract ALiquid superclass represents the union of Coca Cola, sulfuric acid and milk. That is,
a superclass represents the union of allof its subclasses.
or in other words
a superclass represents all that is abstractly equivalent about its subclasses.
For instance, the notion of an abstract liquid embodies all the behaviors and attributes such as having no definite shape, randomized atomic positions, freezing and boiling points that are common to Coca Cola, sulphuric acid and milk. Note the fine distinction between having a value and having the same value.
The above diagram illustrating the relationship between a superclass and its subclasses is called the Union Design Pattern. The union pattern shows us inheritance in how the Coca Cola, sulfuric acid and milk will all inherit the abstract behaviors of liquids, such as the lack of a definite shape and freezing/boiling points. Conversely, it also shows that if a situation utilizes a liquid, either Coca Cola, milk or sulphuric acid can be used as they are all abstractly equivalent as liquids. Note that this does not imply that all three will act identically! For instance, the human throat can swallow any liquid because it is made to work with fluids that can flow. However, the reaction of the throat to sulphuric acid is markedly different than it reaction to milk! This ability to substitute any subclass for its superclass and get different behavior is called polymorphism.