Humans perceive depth in a seemingly two dimensional world by interpreting binocular visual cues from the environment.  They interpret those stimuli in the brain.  Two depth cues that require both eyes are binocular disparity and convergence.  Your eyes are separated by a space.  This allows them to receive two slightly different images.  As distance from your eyes increases the difference between the two images decreases.  Binocular disparity is the depth cue that is based on the difference between these two images.  Your brain interprets images that are very similar in both eyes as far away, and images that are different in both eyes as closer. 
     Another way for you brain to interpret the distance to objects is known as convergence.  This is when your brain interprets the tension in the muscles that control eye movement.  Binocular disparity is used with images that are farther away, perhaps up to 1000 feet.  Convergence is used when the object is eighty feet away or closer. 
     An artistic use of the human processing of depth perception is utilized in stereograms.  Have you ever looked at a stereoscopic image, or "magic eye"? They are two dimensional images, but they can be perceived as three dimensional images in brain.  The feeling of "depth" that you get by looking at a statue instead of looking at a photo of the same statue, is due to the fact that the human body has two eyes. By having two pictures of the same object, taken by two different positions, which is the case of the human eyes, you can get a "three dimensional image."  In the same way, the eyes forward to the brain two slightly different pictures. It is the brain that must "compute" a 3-D representation of the scene.  The brain can make mistakes in this process of designation of pairs! It is these mistakes which make possible the 3-D feeling that we get from viewing a stereogram. 
     To see some examples of stereograms follow any of the links below,

3D Vision
http://www.vision3d.com/

Magic Eye Inc, 
http://magiceye.com/

3D Sterograms   
http://www.softsource.com/stereo.html 

Stereograms 
http://www.eleves.ens.fr:8080/home/massimin/sis/sis.ang.html 

     Create Your Own at,

3D Stereoman's Online Stereogram Generator
http://members.aol.com/ginbg/3dstereo/

Or go to Altavista at http://www.altavista.com and search for Stereograms yourself.  Enjoy!