Principals of Design

Balance

Balance is created from a work of art when textures, colors, forms, or shapes are combined harmoniously.
This photo shows radial symmetry between faces which are also perfectly symmetrical.  The all over symmetry of the piece balances out.

Contrast

Contrast is the use of several elements of design to hold the viewer's attention and to guide the viewer's eye through the artwork.
The piece shown here shows various shades of grays, with a stark contrast between the darkest and lightest shade.  The grays range from an almost-white to black, providing a great difference in value.

Movement

Movement is the way a viewer's eye is directed to move through a composition, often to ears of emphasis.  
Movement can be directed by lines, contrasting shapes, or colors within the artwork.
Line is the main funnel of movement shown by this picture, as all the diagonal lines lead the viewer's eye toward the top right corner.  The change from a large portion of color to a smaller portion of color all in one angle (as seen in the various triangles) has the same effect.

Emphasis

Emphasis is created in a work of art she the artist contrasts colors, textures, or shapes to direct your viewing towards a particular part of the image.
In this picture a pop of red contrasts with the values of gray that are present in the photo.  The bright color stands out from the grays, so the red triangle is emphasized and that's where the focus is.

Pattern

Pattern is the repetition of a shape, form, or texture across a work of art.
In the photo above, a texture is repeated.  The diagonal lines reaching across the leaves are repeated vertically, and the texture of the smaller veins are shown throughout as well.

Proportion

Proportion is created when the sizes of elements in a work of art are combined harmoniously.
The above picture uses line, symmetry and shape to create a harmonious image.

Unity

Unity is created when the principles of analysis are present in a composition and in harmony.  Some images have a complete sense of unity, while some artists deliberately avoid formal unity to create feelings of tension and anxiety.
While the swirls above do not all match in perfect unison, the same rainbow colors are apparent throughout the image and there is a consistent swirling pattern.