Abstract Classes

A method must be overridden
cannot be instantiated
advantage is that it enforces certain hierarchies for all the sub classes
can provide complete, default code and/or just the details that need to be overridden
advantage is that it allows implementations to share common code

Interfaces

Interfaces
An abstract modifier introduces a new virtual method but does not provide an implementation for it.
An abstract method is only allowed in an abstract class.
The implementation must be implemented in a derived class.
An abstract modifier is also a virtual modifier by default [[copy and paste from classes top level page]]

Interface MyInterface 
{ 
   void method_Name(); 
} 
public class BaseClass : MyInterface 
{ 
   public abstract public void method_Name(); } 

public class DerivedClass : BaseClass 
{ 
   public override void method_Name() 
   { 
      //implementation 
   } 
}