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.