The document discusses inheritance in Java. It defines inheritance as deriving a new class from an existing class, known as the base or parent class. The new class is called the subclass or child class. The subclass inherits all variables and methods from the parent class. There are three types of inheritance in Java: single inheritance where one subclass extends one superclass, multilevel inheritance where a subclass extends another subclass which extends a superclass, and hierarchical inheritance where a superclass has multiple subclasses.