The rules to Sudoku are very simple: fill in the grid so that every row, every column and every 3x3 box contains the digits 1 through 9. There is nothing more to it! There are 75 levels in total, split up into the categories Easy (1-25), Medium (26-50) and Hard (51-75). There is a timer, but no limit, so you can play at your own leisure or put yourself under pressure and challenge your friends.
Use the keyboard to select squares using the arrow keys and then type in the digit you want to place. 0 will set the square to empty. You can also fill in the grid by selecting squares with the mouse and pick digits from the small grid that pops up.