What are the differences between System.String and System.Text.StringBuilder classes?

String is immutable. Immutable means once we create string object we cannot modify. Any operation like insert, replace or append happened to change string simply it will discard the old value and it will create new instance in memory to hold the new value. Example: string str = "dev"; onestr = "endra"; str = "help"; String builder is mutable it means once we create string builder object we can perform any operation like insert, replace or append without creating new instance for every time. Example: StringBuilder sb = new StringBuilder(""); sb.Append("hi"); sb.Append("test "); string str = sb.ToString();