1) Click "Play Sudoku"
2) Select degree of difficulty (easy, medium, hard)
3) On the leftmost number pad, select a digit by typing a number or by clicking on the pad,
4) then click on an empty square on the puzzle to set the number.
A sudoku puzzle usually consists of a nine-by-nine grid. Some of the spaces contain numbers; the rest are blank. Your goal is to fill in the blanks with numbers from 1 to 9 so that each row, each column, and each of the nine three-by-three blocks making up the grid contains just one of each of the nine numbers. Sometimes sudoku is misspelled as sudogo, sudogu, or sudoko.
The minimum number of clues required for a Sudoku puzzle to have a unique solution is 17. The maximum number of clues a Sudoku puzzle can have without having a unique solution is 77. The number of all possible Sudoku puzzles is 6.67×1021.
 Gary McGuire, Bastian Tugemann, Gilles Civario, There is no 16-Clue Sudoku: Solving the Sudoku Minimum Number of Clues Problem, January 1, 2012, arXiv:1201.0749v1, http://arxiv.org/abs/1201.0749v1