Maybe you've heard it called the rule of thirds. But if we all rigidly followed it as a rule, photography would suffer as many photographs that do not follow this rule are masterfull. Used as a guideline, however, it can add interest and appeal to your photographs.
The general idea is to find a pleasing placement for the objects in your photograph, rather than simply placing the main object in the center of the photo, or having your photo appear to be a random collection of elements.
To use the rule of thirds, visualize a 3 x 3 grid, like a Tic-Tac-Toe game overlaying the scene you see in your viewfinder or LCD. When composing your shot, try to locate the main objects in your photo at the intersection of these grid lines. In other words, place your main objects 1/3 up from the bottom and 1/3 over from the left, or 1/3 down from the top and 1/3 over from the right. This goes for the horizon line as well. It's suggested placing the horizon right through iether the upper or lower line.
Photographs composed according to the rule of thirds tend to appear balanced and dynamic. Often, depending on the theme of your photo, that may not be what you want: you may want a little tension, for instance. This can often be achieved by moving the elements a little further off of the grid intersection points.