What is The Rule of Thirds?

What is the Rule of Thirds?  It is a "rule of thumb" for composing visual images.  The Rule of Thirds 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.  In the simplest terms, try to avoid staging your character dead centre in the middle of the screen.