Where you place your subject in the photo is what the rule of thirds dictates. Often the subject is placed right in the center by amateur photographers. Occasionally this works, but a different position is often better.

The rule of thirds breaks up your photo into nine parts. Imagine a photo being sliced up by two equally spaced horizontal lines, and two equally spaced vertical lines, creating nine equal sized segments. There are also four locations where the lines intersect.

To use an example, let's say you are photographing a solitary tree in a field, while a long way off in the distance is the horizon. Most amateur photographers put the tree and the horizon smack in the center of the picture. Instead, using the rule of thirds we're going to move them somewhere else.

The tree will be put on one of the points of intersection between a vertical and horizontal line. This photo would look great with a large sky behind our tree, so we will want to use one of the lower intersection points as the position of the tree. The horizon will also want to be along the lower horizontal line, so that there is two thirds sky, one third field.

This positioning of the tree and horizon in this example will add much more interest and 'tension' to use a photographic term, to the image. It will appear more professional than if you had positioned the tree and horizon in the center of the image.

As with all rules of photography, they're not really rules, more like guidelines. So don't think you must use the rule of thirds all the time. Usually it produces a nicely composed, professional looking picture. But sometimes you'll want to 'break' the rule to get an improved composition.

When to use the rule of thirds is something you will learn with practice. I suggest that you try it for almost every picture you take, as generally it works. If it didn't work, just delete the photo and repeat. That's the beauty of digital cameras, you can keep photographing until you get it right!