Balance in design is important. Designers must balance things like unity vs. variety and figure vs. ground. Balance can also be described as achieving visual equilibrium via symmetrical and asymmetrical balance. To do this, designers find balance by choosing what is where in a composition.
Symmetrical and/or asymmetrical balance are as important in painting and photography as they are in a typographic layout. It is imperative that designers understand the pleasing visual of balance while also controlling the visual hierarchy in a dynamic layout.
"Symmetrical balance is also called formal balance because a form (formula) is used. This formula is the usage of a mirror image about a vertical (or horizontal) axis. The results look formal, organized and orderly. There is a strong emphasis on the center axis in symmetry since all of the information is reflected from this point...Symmetrical images have a strong sense of unity because at least half of the image is repeated. At the same time they lack variety because only half is unique."
"Asymmetry means without symmetry. That by itself has nothing to do with balance. It just means that there are no mirror images in a composition. The term, however, is usually used to describe a kind of balance that does not rely on symmetry: asymmetrical balance.
There is no simple formula for achieving balance in asymmetrical balance (hence the term informal balance) so the designer must sense whether or not the composition is balanced. The composition either looks like it is balanced or it does not. Where does your attention go when you look at an image? If it seems to wander around more or less evenly, there is probably balance. If you seem to always come back to the same area before looking elsewhere, then the balance is suspect."
Text paraphrased from http://daphne.palomar.edu/design/bsymm.html
Consider the asymmetrical balance shown in the Power of Thirds image from a couple of weeks ago or in a page layout regarding Grid Theory from last week.