This site describes balance as "the concept of visual equilibrium." It can be either symmetrical or asymmetrical. Symmetrical balance has equal "weight" on either side of a fulcrum down the center of the page. Sometimes this could be a mirror image or "approximate symmetry," in which the content is not identical on either side but are equivalent in a way that maintains balance. Asymmetry, on the other hand, involves arranging objects of different visual "weight" so that they balance. This could be achieved by a lot of small objects on one side that balance one large object on the other. The fulcrum can also be moved around the page. I like this site because it offers examples that can be enlarged by clicking on them.

My Popplet mindmap.