Gestalt Theory

Gestalt Theory (Wikipedia) is more of a description of human vision than a true set of rules.  The Theory says that humans perceive the whole before the parts.  

Emergence - Objects close together become one.  

We don't see legs, a tail, a head and fur, rather we see a dog.  It is only later that we see these individual parts if we chose to study the object.

The last time you went to the store, what kind of shoes was the clerk wearing?  You probably don't know, but you know she was wearing shoes.  Your brain handles so much information in a day that it simplifies.  You only would have noticed the unusual (no shoes).

Reification - Visual construction of non-existant objects

Your mind tends to fill in the blanks.  You can imagine the clerk wearing shoes even if you couldn't see them.

This is a popular trend in design right now.  Schilling Bros. Lumber is a great example.  By taking a letter and hanging it off the edge of the design, your mind creates the rest of the "S".

Multi Stability - One image has multiple interpretations

M.C. Escher made a career out of Multi Stability.  The mind will shift between the 2 or more versions of what it sees.  This is not often used in commercial design, but can be used subliminally.

Other famous versions of Multi Stability are the 2 face/vase and the old/young woman.

Invariance - Ability to recognize objects even when they vary

Even if you only saw a red car from the front, your mind can recognize it as the same car when you see it from the side or if it is now white.  Your mind can recognize most familiar objects no matter how they are rotated, skewed, colored, deformed, or even redrawn in a new style.