msdn上的一个例子看不太懂，请高手讲解一下，关于virtual,new,override的

/*

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如果你安装了 msdn for visual studio 2005, 下面这段代码可以在这里找到：

ms-help://MS.MSDNQTR.v80.chs/MS.MSDN.v80/MS.VisualStudio.v80.chs/dv_csref/html/323db184-b136-46fc-8839-007886e7e8b0.htm

*/

using System;

// Define the base class

class Car

{

public virtual void DescribeCar()

{

System.Console.WriteLine("Four wheels and an engine.");

}

}

// Define the derived classes

class ConvertibleCar : Car

{

public new virtual void DescribeCar()

{

base.DescribeCar();

System.Console.WriteLine("A roof that opens up.");

}

}

class Minivan : Car

{

public override void DescribeCar()

{

base.DescribeCar();

System.Console.WriteLine("Carries seven people.");