Photographers use all sorts of tricks and techniques to make their pictures. Some are technical and others are creative, but they all work toward the same goal, of trying to get others to stop and look at the picture. And if someone looks, you want them to see immediately what the subject of the photo is. One way to do that is to use framing elements, which are simply compositional elements that help guide the viewer's eye to the main subject in your photograph.
Think of it this way, when you point your camera at a subject and look through the viewfinder, you're making a choice about what you want to show. The act of including or excluding things is a compositional choice that starts with framing of the image.
At its simplest, framing elements are usually darker areas that help guide the eye to what you want it to see. Classic framing elements that are often used are windows and doorways. Other objects—leading lines—lead the viewer's eye through an image can be used to frame your subjects as well, such as paths or roads, railings and fences, power lines, rows of trees and bushes, and railroad tracks. Looking at photographs that utilize such framing elements, you can see how your attention is grabbed by the framing element or leading line and brought through the image to the main subject.
Framing elements can be used in other ways as well. If you've found a scene you like and it includes a lot of sky, the viewer's eye could be pulled away to that open area. And if it is a blue sky full of big fluffy clouds, it will be even brighter, causing more of a problem. In that case, look for something to include in your framing of the shot that will be darker and help block some of that sky. Trees or branches can be great for that, especially if they have leaves.
Leading lines, on the other hand, are not always as obvious as a framing element, but they work just as well to bring your eye through a photograph. Leading lines can be straight or curved, but they are always dominant elements that cause you to follow them through a scene.
A leading line should lead your eye through an image, and, like most framing elements, should begin in the foreground of a photograph, but not necessarily a foreground object that takes the focus of the image off the main subject.
The direction your subject is facing can also be a framing element, if the subject takes up a large part of the frame and is facing into the frame. When the viewer's eye follows the subject's line of sight, they will be brought into the photograph.
There are also times you look for framing elements to add interest to a shot. For instance, if there's a nice full moon, it might be too small an object in the sky, however, if you can find something else to include in the shot, such as a rock wall, a tree, a cliff or a person, suddenly you're adding not only framing, but also an element of interest. These can even be silhouettes, as its more the shape than the detail that is adding the interest to the picture.
When composing images with framing elements, you want to keep a few things in mind-make sure your camera focuses on the subject, not the framing element; and if shooting from a darker area to a lighter area (i.e. through a window), remember that it's the outside area you want exposed properly, not the inside. In that case, you may need to utilize exposure compensation (such as -1.0), or spot meter on your [exterior] subject.
Once you start looking for framing elements and leading lines you'll find them everywhere. And then you'll find that they can really improve your photos.