You might find killer sudoku puzzles easier or harder than regular sudokus. They certainly make a fun change. In addition to the regular sudoku rules, each of the outlined areas must add up to the total given for that area.
So in our example (a 6 by 6 grid), each of the rows and columns must contain one and only one of the digits from 1 through six. But the thing that makes it killer, are those little numbers in the left corners of each of the red outlined areas. The digits in a given outlined area must add up to the little number for that area.
This should get your mind thinking in some interesting new ways about numbers!
If you get stumped, the answers are on the second page of the printable PDF's.
You can get started on solving these puzzles by first training with regular sudoku. We have a collection of free printables just for beginning solvers here: Easy Sudoku Puzzles.