classes

Check this.

http://www.dotnetperls.com/class

http://www.csharp-station.com/Tutorial/CSharp/lesson08

Take a look at this example:

using System;

class Base
{

}

class Derived : Base
{

}

class Program
{
   static void Main()
   {
      Base b = new Base();
      Derived d = new Derived();
      Base c = new Derived();
      Derived e = new Base(); // compiler error
      Derived f = (Derived)c; // OK as c refers to a Derived object
   }
}

Note that instances or objects of a class are different names for the same thing.

Note also that you can assign a reference to a Derived object to a variable of type Base but not the other way around.