Inheritance is the powerful mechanisam for resuabiltiy.
We can derive a class from whereever and whenever.
The resuabiltiy avoids to rewrite code and code length.

Inheritance is one of the OOPS concepts(the OOPS concepts 
are Abstraction,Encapsulation,Polymorphism and Inheritance).
Java is purely a OOP language.

Inheritance is the ability of a class to use member data 
and methods of another class.This is done by deriving a 
class from a diff class.The class from which it derives is 
called the base or the parent or super class and the class 
which derives is called the derived or the child or sub 
class.

Java supports two types of inheritance SINGLE inheritance 
and MULTILEVEL inheritance."extends" is the keyword to 
inherit properties of the base class.All the derived 
classes having a single base class is termed as the single 
inheritance.for example
class A
{
...
}
class B extends A
{
...
}
public class C extends A
{
...
}
this is single inheritance

whereas, a class inherits the properties of another class, 
for example
class A
{
...
}
class B extends A
{
...
}
public class C extends B
{
...
}

java doesnot support multiple inheritance.However, a class 
can implement any number of interfaces(interface is a 
collections of methods with no definition(abstract methods))

Inheritance is a form of software re usability in which new
classes are created from existing classes by absorbing their
attributes and behavior using the key word 'extends'.
The class which absorbs the properties is called 'sub class'
and the class which is used is called 'super class'.
Ex:
class Room
{
  void roomAttributes()
  {
    int roomlength;
    int roomheight;
    int roombreadth;
    String roomname;
  }
  void roomBehavior()
  {
    System.out.println("Two windows");
    System.out.println("One 6.2*3.5 door");
  }
}
public class BedRoom
{
  public static void main(String[] args)
  {
    Room roomobj=new Room();
    roomAttributes();
    roomBehavior();
    int area= room.length*room.breadth*room.height;
  }
}
In the above example the room is a super class from which
bedroom(subclass) is inheriting the properties and
attributes. This is simple inheritence.
Similarly, multi level inheritence is also possible.

Ex: class A extends B
    class B extends C
    class C extends D

But, java does not support multiple inheritance.
Ex: class A extends B, c  

For better reference : Deitei & Deitel or Balaguruswamy

inheritance is the process of inheriting the properties of 
super class to sub class