what is the Difference between String and Stringbuilder in C#, Asp.net

what is the Difference between String and Stringbuilder in C#, Asp.net

string str = "hi"; // create a new string instance instead of changing the old one str += "test"; str += "help";

StringBuilder sb = new StringBuilder(""); sb.Append("hi"); sb.Append("test "); string str = sb.ToString();