Visual Hierarchy is a principle of page design by which elements are presented in ways that visually suggest their relative importance, the order in which they are to be noticed, and the intended flow of attention.
The lack of visual hierarchy creates a sense of competition and chaos.
Verbal Texts create a sense of visual hierarchy with variations of type size, white space, and the top-down-left-to-right rules of readng. (From CSS Zen Garden)
Visual Texts create visual hierarchy through composition, flow, and a knowledge of how the human eye tracks through a visual field. (See a full-sized version of El Lissitzky's "The Constructor."
Web pages create visual hierarchy by attempting to combine the logic of textual reading with the principles of visual composition. (See the Avery Island page from Tabasco's web site)