This page contains sudokus, but they are slightly different from regular sudokus. This is how sudokus work:

Every row, every column and every box shall contain the numbers 1-9. (Boxes are the ones which are 3x3 squares.) For obvious reasons no number can occur more than once in a specific row/column/box, since they contain only nine squares.

These are the kind of clues you can get:

If a square looks like this, you know it contains the number written in it.
If a square looks like this you are not given any clue regarding what is in it. It could be any number from one to nine.
If a square looks like this you know it does not contain any of the numbers which are covered. In this case the square does not contain eight or nine.