VirusDog is a tile-matching game which requires you to engage your brain and try to think many moves ahead. The task before you is to clear each successive grid of "virus" tiles and you move a cursor to any tile, press ENTER and then fling it in any direction. If it hits a tile which is identical, both tiles disappear.
The difficulty is that tiles can also be inverted if they collide with a different type of tile. An inverted tile needs to be matched with an identical tile or inverted again by throwing it into another to put it back as it was. If this sounds difficult to follow then just play the game - it quickly falls into place.
As puzzle games go, VirusDog is reasonably taxing, with a steady increase in difficulty balances with the game's guarantee in that all of the grids can be solved in six moves or less. If you like reversi, mah jong and chess then this is going to be pretty addictive fair.