Sudoku rules:

For the most common type of Sudoku (n=9), as displayed in figure 1, there are 9 sub-squares with 9 fields each. Every row and column has 9 fields. To solve a Suodoku game, nine different numbers (from one to nine) have to be placed within the fields so that the three following rules are not broken:
In each row no number is allowed to appear twice
In each column no number is allowed to appear twice
In each sub-square no number is allowed to appear twice
As soon as all different numbers are placed exactly once in each row, column and sub-square, the Sudoku riddle is completed. Figure 2 shows examples of allowed and forbidden steps in a Sudoku grid.
Depending on the degree of difficulty a certain amount of numbers is given when a game is started.

Sudoku-Killer Manual:

In Sudoku play mode, which is the default mode when entering www.sudoku-killer.com, there are six different buttons enabling user interaction with the sudoku grid: new game starts a new Sudoku game in a grade of difficulty chosen by the user in a menu opening as soon as the button is pressed. The more difficult the Sudoku game, the less fields are predefined, such that the player has a harder time placing numbers without breaking one of the three basic save gives the possibility of storing the current Sudoku grid with all inserted numbers as long as the website is kept open. Up to four games can be saved. load retrieves a stored Sudoku game from memory and displays it in the Sudoku grid. Of course this option requires precedent use of save. hint is an option to help the user solve a Sudoku riddle. Clicking this button adds one correct digit within the grid at a random position. If the game does not have a valid solution, this information is displayed. surrender can be used as a last option. The solution to the current Sudoku game is displayed, ending this round. If the Sudoku cannot be solved due to a wrongly positioned digit, this information is displayed. In this case the right solution can be displayed be first clicking restart and then surrender. restart deletes all user defined numbers from the grid, so that the player can begin the current game from scratch again. In solve mode the user can type in a Sudoku riddle into an empty grid to make the sudoku-killer solve the riddle. There are two options available in this mode: solve calculates and displays a solution for the unfinished sudoku the user types in, as long as the Sudoku has a valid solution, of course. clear deletes all fields, enabling the user to insert a new Sudoku riddle easily.