The rule of thirds is a "rule of thumb" or guideline which applies to the process of composing visual images such as designs, films, paintings, and photographs. The rule of thirds is used to draw the viewer into the image, instead of just looking at the centre. The picture is divided into nine smaller squares, which helps the photographer to find the horizon and focal points of the subject being photographed. By placing focal points at each intersecting line, the picture/shot is automatically made more interesting. It also helps to balance images, and prevent shots from having all of the focal points in one area. 

The photographer aligns the subject with the guide lines and their intersection points, placing the horizon on the top or bottom line, and allowing features to flow from section to section.

The main reason for observing the rule of thirds is to stop the placement of the subject at the centre, which makes the shot look flat and uninteresting. If filming a moving subject, the same pattern is often followed, with the majority of the extra room being in front of the person (the way they are moving). Likewise, when photographing a still subject who is not directly facing the camera, the majority of the extra room should be in front of the subject with the vertical line running through their perceived centre of mass.