Sudoku is a logic-based, combinatorial number-placement puzzle. In classic Sudoku, the objective is to fill a 9 × 9 grid with digits so that each column, each row, and each of the nine 3 × 3 subgrids that compose the grid (also called "boxes", "blocks", or "regions") contain all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a single solution. Source:
Wikipedia
How to play?
Select with MOUSE_CLICK, change selected field with ARROWS.
Enter a number in selected field by pressing NUMBER_KEY (comment mode has to be switched off).
Commenting: while COMMENT_MODE is switched on, select a field you want to add comments to and press a number key (press it again if you want to remove comment).
Safe mode: prevents the user from entering a number which already appears in current row, column or block.
Cancel last move with BACKSPACE, erase any selected number with DELETE.