In this illusion, it appears that there is one set of black figures and one set of white figures. In fact, the two sets of figures are exactly identical. They appear different because the surrounding regions they are on cause the visual system to segment the images into layers. Thus one set appears to be white figures behind dark clouds, and the other set appears to be dark figures behind light clouds. If you cut out the figures you will see that they are identical!
Image segmentation and lightness perception Barton L. Anderson & Jonathan Winawer Nature. 2005. 434:79-83