Would you like to take more impressive photographs with a minimum of effort? Good composition is the key and the "rule of thirds" is the easiest technique to apply ... but it's surprisingly tricky to master. Beginning photographers tend to put the subject right smack in the middle of the frame. This is perfectly natural because when we look at an object, we center our eyes on it. However, to compose a photograph, you must mentally step back and consider the entire image.

See how this subject is centered in the image? This is how we view the world so it's very likely we'll start out by shooting photos like this one.Now, imagine dividing the scene in your viewfinder into thirds, both horizontally and vertically. The rule of thirds has you mentally superimpose this grid upon your scene and move the subject so that it sits on one of these lines.
Here we see the image with a grid upon which to place the subject. Adjusting the composition so that the subject falls on one of these lines creates a more interesting photograph than a centered image. Note how the subject is really not that far from the center so it requires very little effort to recompose! Placing the subject on the intersecting lines creates a much stronger composition.

You notice how this technique creates nine rectangles within your frame? Ignore them! The rule of thirds is about the lines, not the rectangles. It's a common misconception that those rectangles are used for something ... they're not.

The rule of thirds is particularly useful when photographing horizons. The typical beginner's landscape has the horizon bisecting the image. With a slight composition adjustment, the horizon is pushed up or down depending on the effect you want to achieve.

The horizon falls on the bottom third line making for a very dramatic sky. The distant farm is placed on the right vertical third line making the composition that much stronger.
Aligning the horizon along the top third line draws the eye through the greenery to the misty horizon. The image would not be nearly as dramatic had the composition been centered.

The closer you are to the subject, the more challenging it is to apply the rule of thirds. Full length portraits, for example, require you to place your subject to one side of the center of the image.

In this full-length portrait, the subject stands on the left third line while his face falls on the top third line. The large open space to the right helps set the scene.
With a close-up portrait, the subject becomes the eyes ... which can fall nicely on the vertical third lines and top third line.
Placing the subject's face on the top and right third lines provides viewers with more context about the image, drawing them in. Having the subject on the right side is often quite pleasing to people who read from left to right. Doing so causes them to scan across the background before arriving upon the subject.

Some cameras provide a 3x3 overlay grid on the viewing screen to enable photographers to easily use the rule of thirds. Although it's easy to imagine the grid, using such a visual aid will serve as a reminder to compose the image.

There's nothing wrong with applying this technique when you crop the image in your editing software, but by training yourself to compose during the shoot, you will find yourself considering other composition methods as well.

Some people get hung up on the "rule" part of the rule of thirds, insisting that rules are made to be broken or some such nonsense. This composition technique is an excellent way of improving your images, but it is not an immutable law. There are many circumstances where you can not, or perhaps should not, apply the rule and must rely on another method of composition.

Attempting to shoot an image using the rule of thirds is certainly a challenge as you must consciously re-frame the scene. If you habitually center your subject, consciously apply the rule of thirds and your images will improve dramatically. The most difficult part for beginning photographers is to think about and apply any composition technique, so starting with this one is an excellent choice. Once you become aware of your composition, you will find yourself considering other techniques that will further improve your image.