The Rule Of Thirds is a guideline which applies to the process of composing visual images such as designs, films, paintings and photographs. To follow this rule you have to imagine an image that is 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.
By using this technique it creates more energy, tension, and interest in the composition than simply centering the subject would.
Here is a video demonstrating how the Rule Of Thirds is used.