Start a new game and you get a grid with the usual rules: every row, column and box holds one through nine exactly once. Click a square, type a number, and the board tells you nothing you have not earned. Difficulty is chosen at the start, and it is a real difficulty rather than a label.
The whole thing is one screen with no timer running and no advertisement between puzzles. That is most of the reason to use it. An easy grid runs about ten minutes, a hard one runs half an hour and involves at least one moment of putting a pencil mark in three squares and walking away from it.