The objective is to fill all the correct circles based on the information given in each of the squares. The squares tell how many circles around it are filled. Using your ninja powers of deduction, fill in all the right squares to win. It doesn't get any simpler than that.
A variety of puzzle sizes and difficulty levels are available to tailor your game play experience to match your own personal comfort level. It's a no-frills design that minimizes the superfluous to maximize what it does well: classic puzzle gameplay.
Update: The game's author just updated SquarO by adding support for "negative markers" (for when you know a circle should not be filled. To mark circles in this way, use CTRL-Click (on Mac, use Command-Click). Very nice! :)
Cheers to Tryss for suggesting this one. =)