[asadullah.ansari]

You can search on google for detail Or ask some point where you are unable to understand.
In short:

In c++ two types of polymorphism One is Compile time : By function overloading,operator overloading and template also.

and second is Run Time polymorphism: by Virtual function

>>> By the use of virtual function, we can override the function ( b/w base and derived class)

class base
{
  public:
   void Fun()
  {
    cout<<" I am from base"<<endl;
  }
}


class derived: public Base
{
   public:
    void fun()
    {
         cout<<" I am from derived"<<endl;
    }
}

int main()
{
   Base *bptr;
   Derived dObj;
   bPtr=&dObj;
   bPtr-> fun()
}

Output will be
I am from Base
Because It will never call to derived because No run time polymorphism support here.

If you write virtual keyword before function then

class base
{
  public:
   virtual void Fun()
  {
    cout<<" I am from base"<<endl;
  }
}


class derived: public Base
{
   public:
    void fun()
    {
         cout<<" I am from derived"<<endl;
    }
}

int main()
{
   Base *bptr;
   Derived dObj;
   bPtr=&dObj;
   bPtr-> fun()
}

Output will be
I am from Derived
Because It will call to derived because run time polymorphism support because of writing virtual.

For detail you can see this thread.
http://www.go4expert.com/showthread.php?t=8403