The Rule of Thirds is a guideline that helps you better frame and composite your shots. It states that you should imagine every image as being divided equally by 2 horizontal lines, and 2 vertical lines. The important parts of the image should fall on those lines, or on their intersection. The main goal of this rule is to prevent you from shooting everything dead in the center of your frame. Following this guideline will make for more interesting shots, rather than a boring centered image.
Continue reading the full tutorial here – http://www.rodypolis.com/15/post/2012/06/ruleofthirds.html