object vs reference type

Question

 Consider we have follow scenario

Class base

{

          public void print()

          { 

          Console.writeline("Print from base class");

          } 

} 

 

Class derived : base

{

          public void print()

          { 

          Console.writeline("Print from base class");

          } 

} 

 

and in the main method if we write like this 

main()

{

      base b = new derived();   // My 1st question is what happens at this point in time

      b.print(); 

}                      //Now then why do we need to write like this we could declared it as "base b= new base();"  because anyway we can't call child class variables from b.

//also somebody pointed out that  "base b = new derived();" in this case b become object of child class to a reference variable of base type.

//I want to know what does this mean.  

 

Please suggest with your observations.

 

Thanks in advance