The rule of thirds is a "rule of thumb" or guideline which applies to the process of composing visual images such as paintings, photographs and designs The guideline proposes that an image should be imagined as divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections.Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject would.

http://en.wikipedia.org/wiki/Rule_of_thirds