Retris is an interesting and highly polished 'reverse Tetris' puzzle game. When the game starts you'll see one single tetris block falling down the well on the left. On the right-hand side are a bunch of other pieces. The aim is to use your mouse to drag the pieces on the right over to the left-hand well, and position them all together so it creates a complete block. The piece falling from the top is the 'cherry' on the cake, and should fill the remaining space in your otherwise solid block of tiles.
Get it right and all the solid lines clear and you get lots of points. Mess it up and an immovable set of tiles appear at the bottom of the well, filling up your available space for the next round.
It takes a little while to get used to the concept, but once you've nailed it, it becomes incredibly addictive.