What is an interface and what is an abstract class? Please expand by examples of using both.

Abstract class-
An abstract class is a special kind of class that cannot be instantiated. So the question is why we need 
a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In 
other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that
forces all the subclasses to carry on the same hierarchies or standards.


Interface:
An interface is not a class. It is an entity that is defined by the word Interface. An interface has no
implementation; it only has the signature or in other words, just the definition of the methods without
the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies
for all subclasses or it defines specific set of methods and their arguments. The main difference  between them is that a class can implement more than one interface but can only inherit from
one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement
multiple inheritance.