Sudoku puzzle explanation

Sudoku is a diagram of nine by nine boxes in which the numbers from 1 to 9 must be entered. The diagram of 9x9 is again subdivided into a box of 3x3. A number from 1 to 9 must be entered in each box.
In order to solve the puzzle, the boxes must be filled in such a way that in each row, every column and each box of 3x3 the numbers 1 to 9 only occur once.

You do not have to be a clever head, just a little patience and logical reasoning is needed to solve the puzzle.