Rule of Thirds
The rule of thirds is one of the most important rules of photographic composition. Landscape photographers are particularly fond of this one, but it works well for many types of subject.
The rule of thirds simply says that, instead of placing the main focus of interest in the center of the frame, which makes for a very static composition, that you look to position it on an intersection of the thirds. That is to say one third up and one third in or two thirds up and one third in etc.
Placing the Horizon on the Third
The most common application of the rule of thirds is in placing the horizon in a landscape picture. Take a look at the two versions of a landscape picture on the left, I hope you'll agree that the bottom photo is a more satisfying composition.
The sky contains very little detail so why show too much of it? In the bottom picture we have enough sky to show that it is a sunny day and now we have more room for interesting subject matter. The top picture looks cut in half, we feel unsure what we are supposed to be looking at when the picture is cut in half by the horizon.
If we had some interesting clouds to look at, we could just as easily place the horizon one third from the bottom of the picture to show that we are most interested in the sky. But we still wouldn't place it in the center of the frame. Never, ever put the horizon in the middle.
Also notice, in the pictures above, how the tree takes on more importance in the picture on the right because it now sits on the intersection of the vertical and horizontal third, which is a very powerful position in the frame.
Here's another 'thirdsy' sort of picture on the right. Placing the boat near the top of the picture tells the viewer that the main subject of the picture is the reflection in the water.
We could take the boat out altogether, of course, this would focus our attention even more on the reflection but the picture might then be a little too minimalist.
Also the mast is almost exactly on the 'third' line making it a very prominent part of the composition.
Placing the main elements of your composition on the thirds, and especially at the intersection of the thirds, is a powerful composition aid and will immediately improve your compositions.
Try it with every subject, not just landscapes.