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