How to play Sudoku?

When you begin the puzzle, some of the grid's squares already contain numbers - you cannot change these. To solve a 9x9 puzzle, you need to fill every empty square with numbers 1 through 9.

The puzzle is solved when each ROW, each COLUMN, and each 3 x 3 BLOCK (3 x 3 blocks are heavily outlined for easy identification) contain numbers with each number appearing ONCE.

You may type your entries and use keyboard arrow keys to navigate. Or you may use the mouse to click a square, hold the mouse button down, and drag up or down.

Lights on the board turn yellow when a row, column, or 3 x 3 block is filled.

What's Sudoku

Sudoku is a logic-based, combinatorial number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid (also called "boxes", "blocks", "regions", or "sub-squares") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which typically has a unique solution. Completed puzzles are always a type of Latin square with an additional constraint on the contents of individual regions.

Source: http://en.wikipedia.org/wiki/Sudoku