String 和 StringBuilder 有什么区别？

using System;
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
long time1 = 0;
long time2 = 0;
Stopwatch s = new Stopwatch();
s.Start();
useStringBuilder("IDNumber", 20000);
s.Stop();
time1 = s.ElapsedMilliseconds;
s.Start();
useString("IDNumber", 20000);
s.Stop();
time2 = s.ElapsedMilliseconds;
// 在进行大量运算的时候String比StringBuilder需要耗费更多的内存和CPU，而且运行速度也相对较慢。
Console.WriteLine("UseStringBuilder: " + time1 + " milliseconds");
Console.WriteLine("UseString: " + time2 + " milliseconds");
Console.Read();
}

static string useStringBuilder(string ID, int number)
{
StringBuilder builder = new StringBuilder();
for (int i = 0; i < number; i++)
{
builder.Append(ID + i.ToString());
}
return builder.ToString();
}

static string useString(string ID, int number)
{
string temp = "";
for (int i = 0; i < number; i++)
{
temp += ID + i.ToString();
}
return temp;
}
}
}