This tricky visual is called "The Coffer Illusion." What do you see?
There are in fact 16 circles sitting right there in plain sight. Keep staring and eventually you'll see them, resulting in an odd feeling of satisfaction and/or a splitting headache.
According to Digg, here's why we can't see the circles at first:
The Coffer Illusion plays on the fact that the visual brain is heavily geared towards identifying objects. "Pixels" are grouped to form edges and contours, shapes, and finally objects.
Sometimes, as in the Coffer Illusion, there is no "right" grouping because the image is inherently ambiguous. Two different groupings make sense — a single set of horizontal lines can either form a circle, or be the intersection between two rectangles.
For most people, the grouping into rectangles initially dominates. This may be because rectangles (including the ones we see in door panels) are often more common than circles in our daily environment, and so the brain favors the grouping that delivers rectangular shapes.