Simply stated, the Rule of Thirds divides your image into three columns and three rows. Just like a tic-tac-toe grid.

The focal points of the image are where the lines intersect. By placing your subject at or near one of these intersections you can add interest to your image. Now, while it is called the Rule of Thirds it is still only a suggestion. By that I mean, if your composition calls for breaking the rule, by all means do so. You as the photographer are the artist and the ultimate judge of how your image should be composed.

Let's look at the Rule in practice!

Here is a picture of a seagull. As you can see, it looks like he is flying and has some space to fly to.

That interest and sense of movement and space it enhanced by my choosing to crop the photo so as to place the bird at one of the intersections.

Our eyes are drawn to the seagull and we sense that he has some place to go with plenty of room to do it. If I had instead cropped the photo placing the seagull right in the center the image loses some impact.

Now he looks cramped.I don't get the same sense of movement from this subject in the center of the frame. Rick Sammon , renowned photographer and author, has a saying that "dead center is deadly" and in most cases he's right! Of course if there is a subject that just begs to be placed in the center and you feel that is the best composition go ahead and break the rule!

Aside from the four intersections you can use the horizontal and vertical lines as placement guides. For example, in the case of a landscape shot try not to let the horizon line fall at the center and cut the image in half. If the sky is more interesting, place the horizon at the lower horizontal line. If the subject is the land or water, place it at the top line.

In this shot I wanted to emphasize the silky ripples on the lake surface and the reflection of the colors of sunset. The sky was relatively boring so I set the horizon at the top line letting the lake fill two thirds of the frame.

So keep the venerable Rule of Thirds in mind the next time you shoot. Remember that you can crop in Photoshop to arrange your image on one of these lines or points. I've included a free Photoshop Action that places guides at the Rule of Third points so you can experiments. Just load it into Photoshop and run it against a file before you start working on it. Make sure Guides are visible or you won't see them.