Balance is a state of harmony of visual elements within an image. It strives to create an equilibrium of tone, weight and shape within an image on both sides of a vertical or horizontal axis.
Balance applies to the totality of the image and how portions of the image fill the frame in a pleasing manner. Balance is not symmetry. An entirely symmetrical image is often balanced, but it is often boring as well.
See additional information at Address basic composition and the blog PhotoJournalism.