Media Glossary : Rule of Third...

This is the rule of third always you to create a more interesting, balanced and powerful composition by placing the subject off-centre instead of in the centre of the frame. The basic idea behind the rule of thirds is to break down the frame into vertical and horizontal thirds, creating nice equal squares. The reason why the rule of thirds apply is because studies have shown that the human eye naturally sees one of the four intersecting points,  therefore it makes sense to place your subject at one of those four points. This clip explains the basic elements to the rule of third, and why we use the certain shots.