Rules of Perceptual Organization

Closure- Objects grouped together are seen as a whole.

The triangles grouped together in this picture are seen as a whole closure.

http://psychology.about.com/od/sensationandperception/ss/gestaltlaws.htm

Continuity- Lines are seen as following the smoothest path.

The dots seem to folllow the nlight blue path due to it's smoothness, and continuity.

http://psychology.about.com/od/sensationandperception/ss/gestaltlaws.htm

Similarity- Items that are similar seem to be grouped together.

The circles and dots are grouped seperatly due to their alikeness, and similarity.

http://psychology.about.com/od/sensationandperception/ss/gestaltlaws.htm

Proximity-Nearness In space, time or relationship

These sprees are in various PROXIMITIES to each other 

Figure-Ground perception- generally knows as seeing a picture in the negative space of a picture. 

(The most famous example is this:

http://psychology.about.com/od/findex/g/figure-ground-perception.htm

If you looks at the white part of the image, you see a vase, if you look at the black areas, you see two faces looking at eachother.