The object of Sudoku is to fill the other empty cells with numbers between 1 and 9 (1 number only in each cell) according the following guidelines:
1. Number can appear only once on each row:
2. Number can appear only once on each
    column:

3. Number can appear only once on each
     region:

A summary of these guidelines would be, that a number should appear only once on each row, column and a region.