Ask any programmer, he'll immediately reply saying "A destructor is a member function of a class, which gets called when the object goes out of scope". This means all clean ups and final steps of class destruction are to be done in destructor. A virtual function is something which helps a derived class in overriding the implementation of a functionality of a base class.

The order of execution of destructor in an inherited class during a clean up is like this.
1. Derived class destructor
2. Base class destructor

A difference between a destructor (of course also the constructor) and other member functions is that, if a regular member function has a body at the derived class, only the version at Derived class gets executed. Whereas in case of destructors, both derived as well as base class versions get executed.

Now turning our attention to why a destructor has to be virtual, the reason is that we, programmers are very smart. We'll do days and nights of work to inherit and extend the functionality of an existing class which is being used, and say that we don't want to change the implementation/interface just for the sake of a new entrant. Let me explain this with an example.

---

#include <iostream.h>
class Base
{
       public:
          Base(){ cout<<"Constructor: Base"<<endl;}
          ~Base(){ cout<<"Destructor : Base"<<endl;}
};
class Derived: public Base
{
     //Doing a lot of jobs by extending the functionality
       public:
           Derived(){ cout<<"Constructor: Derived"<<endl;}
           ~Derived(){ cout<<"Destructor : Derived"<<endl;}
> };
void main()
{
        Base *Var = new Derived();
        delete Var;
}

Try executing this code, you'll see the difference. To our observation, the constructors are getting called in the proper order. But to the dread of a programmer of a large project, the destructor of the derived class was not called at all.

This is where the virtual mechanism comes into our rescue. By making the Base class Destructor virtual, both the destructors will be called in order. The following is the corrected sample.

#include <iostream.h>
class Base
{
       public:
          Base(){ cout<<"Constructor: Base"<<endl;}
          virtual ~Base(){ cout<<"Destructor : Base"<<endl;}
};
class Derived: public Base
{
     //Doing a lot of jobs by extending the functionality
       public:
           Derived(){ cout<<"Constructor: Derived"<<endl;}
           ~Derived(){ cout<<"Destructor : Derived"<<endl;}
};
void main()
{
        Base *Var = new Derived();
        delete Var;
}

Note:
There is one more point to be noted regarding virtual destructor. We can't declare pure virtual destructor. Even if a virtual destructor is declared as pure, it will have to implement an empty body (at least) for the destructor.