It should be fairly self-explanatory. Just put a number 1-9 into each square.
If you try to put a number in that logically can't go there (there's a duplicate in that row, column or square), then it simply won't let you focus on a new square. (Maybe in the future I'll have it highlight the duplicate).
If you right click on a square, it'll turn the number green and make the square read-only -- handy if you want to mark everything you know is correct and then guess something and follow it to it's logical conclusion.
If you uncheck "Preferences->Hide Numbers", little numbers will show up in each unfilled square. These numbers are the possible values for that square.
For now there are no built-in games, however it will load a new game every day (provided I post one, otherwise it will be totally empty).
I designed it as a "last resort" tool in case I got stuck solving my paper ones.
Send any bugs or problems to me, and I'll see about fixing them! There is a known problem with removing numbers -- the little numbers sometimes don't update correctly, making you think that a number can go in a square when it really can't. If you play with the numbers off, then it's not an issue.