An off-balance work of art can cause anxiety or curiosity. A properly balanced work shows the viewer where to look, or where to look first. Those looking for certain pieces of information will rely on balance subconsciously, and if it is awkward or doesn't make sense to them, they could become frustrated. Balance is generally symmetrical or asymmetrical and can be adjusted using mass or open space.
An interesting design uses contrast -- dark and light, smoothness and roughness, cluttered and empty. All design elements can be used to create contrast. Using this principle is one way to make a certain element or piece of information noticeable. If you want it to stick out, put it in great contrast with the things surrounding it.
Like balance, direction tells the viewer where to look and then leads the eyes through the layout. Direction in based in the design element of line, but shape, space, value, perspective and text can all create direction. They give the illusion of depth, which tells the eyes where to go, what's important, what comes first and what follows. Even if the design is only meant to be looked at, it won't be correctly understood without direction.
Economy is achieved through editing. If the design is cluttered or lacks focus, elements can be removed. If the design still works, you've exercised economy. You don't want to include any more than what is required to produce an economical design. If it can be done with less, too much is a waste of time and makes the product "over-designed."
The emphasis, or message you want to send with your design, should be clear. Balance, contrast and direction all contribute to successfully emphasizing your point. And without an emphasis, your design is forgettable.
Like balance, proportion can evoke emotion. Proportion gives the viewer an idea of what he's seeing and helps him process what's happening in the design. With text, proportion can be used to emphasize quite easily; it creates an order of importance. A few big words on a page seem more individually important than hundreds of tiny ones. Proportion can be used to get viewers' attention or even shock them.
Rhythm is felt in the patterns of the design. Repetition and gradual increase or decrease in value are ways to express rhythm. It can be used hand-in-hand with balance, direction, emphasis and proportion. It can also evoke emotion and create movement. Regular rhythm involves elements separated by intervals, flowing rhythm creates a sense of movement, and progressive rhythm shows sequence. Any way you use it, rhythm keeps the design moving.
Unity is what fits all the elements of a design together. If there's no unity, you may have forgotten to add something, or you may need to delete something. A lack of unity can be confusing and may throw off the emphasis in your message. Simultaneously, unity gives both wholeness and variety. Something should tie your elements together visually to create it.