The Rule of Thirds
The rule of thirds is a guide to help you show your subject off to best effect.
When you look through your viewfinder or at the LCD monitor screen on your camera, it helps to imagine a noughts-and-crosses grid over the scene. The grid segments the image into nine squares, which are created by superimposing four lines over what you can see.
Note that some Nikon cameras even have a menu option that allows you to turn on gridlines in the viewfinder (or on the screen). These gridlines are a guide to help you frame your image and won’t show up in your final picture.
Notice where the four lines intersect. The rule of thirds suggests that these intersection points are the best places in which to position the most important elements of your composition. Doing so will generally result in a more energetic and interesting image.
The subject doesn’t have to be directly on an intersection (also known as a 'power point'). As long as it is close to it, your image will be dynamic and well composed. Try a couple of different compositions to find the one you like best.
These same gridlines can help you to keep your horizons level and the vertical elements in your photo straight.