Eamples on this page 
from The Perceptual  
World and The Optical  
Illusion Book (See Credits)

  Figure ground perception is the tendency to discriminate between target and background stimuli. The stimulus we perceive as being the target is referred to as the figure.  The other stimuli that we do not perceive as the target make up the background, or ground. 
     The vase seen at right can be perceived in two different ways.  If the black is perceived as the background, you will see a vase.  However, if you perceive the vase as the background, you will see two faces.  

     An interesting phenomenon occurs when looking at a square centered over top of several, closely spaced concentric circles.  In the figure at left, all of the sides of the square are perfectly straight.  Go ahead and check with a straight edge.  However, the sides of the square will appear to bend inward toward the center of the circles.  This is known as background mediated illusion. The background is responsible for the distortion seen in the figure.