Inheritance is the mechanism which allows a class A to inherit properties of a class B. We say "A inherits from B''. Objects of class A thus have access to attributes and methods of class B without the need to redefine them.
If class A inherits from class B, then B is called superclass of A. A is called subclass of B. Objects of a subclass can be used where objects of the corresponding superclass are expected. This is due to the fact that objects of the subclass share the same behavior as objects of the superclass.
However, subclasses are not limited to the state and behaviors provided to them by their superclass. Subclasses can add variables and methods to the ones they inherit from the superclass.
In the literature you may also find other terms for "superclass" and "subclass". Superclasses are also called parent classes or base classes. Subclasses may also be called child classes or just derived classes.
Inheritance Example
Like a car, truck or motorcycles have certain common characteristics- they all have wheels, engines and brakes. Hence they all could be represented by a common class Vehicle which encompasses all those attributes and methods that are common to all types of vehicles.
However they each have their own unique attributes; car has 4 wheels and is smaller is size to a truck; whereas a motorcycle has 2 wheels. Thus we see a parent-child type of relationship here where the Car, Truck or Motorcycle can inherit certain Characteristics from the parent Vehicle; at the same time having their own unique attributes. This forms the basis of inheritance; Vehicle is the Parent, Super or the Base class. Car, Truck and Motorcycle become the Child, Sub or the Derived class.