An Abstract Class is one in which the member function(s) at 
the Base(Parent) Class are left undefined, but declared. 
It's upto the Derived(child(ren)) Classes to Implement the 
Functions declared in Base...


Ex:

Let us assume... As a father(Base Class), he has some Land 
(Member Function), and he has not done any cultivation
(implementation) in it... As his Child(Derived Class), he 
takes the Land(Member Function) from the Father(Base) and 
does Agriculture(implementing the M.F. of Parent Class) in  
the Child...

Concept of Inheritance...

Is it good enough!!!

First of all, a class is a software entity to represent a 
concept. If the entity exists in real world, then, we can 
create object of the class which makes sense. This is 
called as a concrete class.

For example, Lion, Dog, Car, Employee, etc.

An abstract class is a class which represents a concept, 
the object of which is not available in the real world. 
This is merely a concept alone. There are object in the 
real world which resembles this concept.

For example, Animal, Vehicle, Person, etc.