In Photography, there is this very famous rule that is preached a lot to the beginners. It's called "The Rule of Thirds".
Hmm... What it is this rule about? Well, in California, The Rule of Thirds means
"Whatever Money you make every month, is divided into Three pieces
First piece is taken away by the Government
Second piece is taken away by your landlord or the mortgage company
And the remaining piece, is shared by you, your spouse and kids."
Just Kidding, :-)
Rule of third is actually about how to position your subject in the picture. Actually it's not even a rule, it's rather a guideline for making reasonably framed pictures.
As there is no "Official Text", here is my version.
"Whenever appropriate, place your subject at one third distance from the edges of your frame."
Take a look at the picture below.
In this picture, the subject - the bikes - is placed in the middle of the frame, little bit off the center. The picture looks OK. Now compare this with the picture below.
The bikes are placed in one corner, centered approximately at one third height from bottom and one third width from right. The picture looks framed better then the first one.
Here is another example.
Below is the same scene, framed again with The Rule of Thirds in consideration.
Main contrast between the two pictures is the fact that, the first picture puts all the emphasis on the surfer by putting him in the middle, while the second picture puts almost equal emphasis on the surfer as well as his surrounding. It also puts emphasis on the direction of his movement and makes it look like the surfer has just entered the frame from the left side.
Another thing worth note here is that the frame is divided horizontally in almost equal parts by water, land and the background of mountains, making it very symmetric.
It does not make sense to try to use The Rule of Thirds for every picture. However, so far I have found that, in any frame where in addition to the subject, you also want to capture an essential property of the surrounding, the rule of third can be used very naturally to create a balance between the subject and the surrounding. For example, the example above shows the beauty of the lake side as well as includes the surfer in harmony with it.