Balance is concerned with the distribution of visual interest. In other words, what is where in a composition. There are two systems for achieving balance:
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 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 there...
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 goes 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, and that is not the center of the composition, then the balance is suspect.
Above text paraphrased from http://daphne.palomar.edu/design/bsymm.html and http://daphne.palomar.edu/design/asymm.html
STUDENTS: ONE SAMPLE EACH OF SYMMETRICAL AND ASYMMETRICAL BALANCE
1.Please post once with a website address displaying symmetrical balance and explain how the sample displays this.
2.Then post again with a website address displaying asymmetrical balance and explain how the sample displays this.
The two do NOT need to be related like the two images above.