System.string works on copy of string data.
System.Text.StringBuilder works on actuval string data

according to me difference between a string class and using String Builder is that in using a string class you need to create new object befour appending it to the main string but in string builder class one don`t need to create object every time

Example for string builder class:
using System.Text;

stringBuilder str = new stringBuilder;
str.append("");

in the previous example once u create str as new instance of string builder class u just have to append every thing in to the string