How is killer sudoku different from normal sudoku?

Killer sudoku has been around for quite a while. Japan has had access to killer sudoku for almost thirty years. There are many similarities between the killer sudoku and regular sudoku. The rules remain the same. For instance, all rows, columns and subgrids must contain nonrepeating numbers between one and nine.

However, one difference is that there is a small number in the corner. The numbers in the rows, columns and subgrids must add up to that number. That makes it more difficult than the regular sudokus. There is actual one level higher than the killer sudokus. It is called the Killer X and there is another rule to that. Besides the rows and columns down, you must also use the numbers only once diagonally across.