The Rule of Thirds
One of the most popular rules in photography is the rule of thirds. It is a technique used to compose photographs. Basically, it is a rule about how to position your subject in the picture. It works like this, imaginary lines are drawn dividing your potential shot into thirds both horizontally and vertical. The result is an imaginary grid, made up of 9 squares (see below).
Using this grid you should place important elements where the lines intersect (see below).
As well as using intersections you can also arrange areas into bands occupying a third or place things on the imaginary line. A good way to place elements is a third of the way up or down the image.
Another is to place your subject a third of the way to the left/right.
A bad way to compose your image is to place things in the middle or in the corner. Using the rule of thirds produces good photographs that are easy on the eye. Below are some good and not so good examples of the rule of thirds.
The image of the tug below looks ok. However, it could be improved.
The image below shows the use of the rule of thirds. Notice how it creates the illusion of movement, the tug moving left.
The image below clearly shows the use of the rule of thirds. The tug has been located where two lines transect. Also, the river is show in the lower third of the image.
The Rule of Thirds by Photospot - Contains good examples illustrating the rule of thirds
Rule of thirds by Photo96 - Again contains some excellent examples