C#의 정적 클래스란 무엇입니까?

using System;

public static class Demo {
   public static float PI = 3.14f;
   public static int calc(int n){return n*n;}
}

class Program {

   public static void Main(string[] args) {
      Console.WriteLine("PI: "+Demo.PI);
      Console.WriteLine("Square: " + Demo.calc(3));
   }
}

public static class Demo {
   public static float PI = 3.14f;
   public static int calc(int n){return n*n;}
}

public static float PI = 3.14f;
public static int calc(int n){return n*n;}