Composition:

Composition, the act of composing the image in the Viewfinder, is a visual process of organizing the elements and individual details of a scene into a balanced and pleasing arrangement. Because what one person finds pleasing, someone else will not, composition is largely a matter of personal taste.

This site shares a great selection of examples of good composition - not only good examples, but a brief discussion about *why* they're well composed.

One of the great benefits of digital photography is that you can improve some aspects of composition after the fact, just with judicious use of the Cropping tool. You can trim out extraneous space, or move the subject over in the frame a bit so it aligns with the Rule of Thirds.

Of course, not everything can be done in post-processing. First, take the best image you can take, then do little bits on the computer after the fact. Some of my favourite images have needed no editing at all...