Rule:
“The rule of thirds is a compositional rule of thumb in photography.

The rule states that an image can be divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines. The four points formed by the intersections of these lines can be used to align features in the photograph. [...] Aligning a photograph with these points creates more tension, energy and interest in the photo than simply centering the feature would.”
http://en.wikipedia.org/wiki/Rule_of_thirds

 
Idea
Composition !
The rule of thirds is widely known by artists, Graphic Designers, and photographers. It is a tool to be used in laying out or composing their pieces in a certain way that would be aesthitically pleasing.

Steps
We will be having an example here how to crop a photograph so that it applies the rule, and then we’ll see how it looks before cropping, and after.

1. Find an image in your photo library.

2. Make a grid of thirds.

3. Align your photo to the grid.

4. Once you like it, crop it.

5. Compare.
The result should be more pleasing to your eye :)

6- Now, once you know this rule, you might not need to crop your image anymore, you can do it right while shooting.

Note: “Rules are made to be broken” as long as you are aware about that, rules can be broken!

Before and After