The rule of thirds is a very basic rule and is pretty easy to remember. Especially when our technology helps us with this. After some apple update my iPad camera now has a grid on it. At first all I could think about was how do I get rid of this, its annoying, and I don't need to be showed how to take pictures. If you're asking what the grids purpose it, its to aide in the use of the rule of thirds.
What is the rule of thirds? Its a way to teach photographers how to line up their subject to create a more balanced image. When you have a grid to work with, the subject of your image would be placed on one of the lines or at a cross section. The center of focus should be where the lines cross.
When you don't have a grid you divide the viewing area into thirds; both vertical and horizontal. Then place the subject on one of the cross sections. Keeping the main focus on the cross section.
The idea behind this is that placing points of interest in those cross sections the photo is more balanced and easier to look at. When looking at photos, your eye will naturally go to one of the cross section, using the rule of thirds works with this natural way of viewing a photo rather than against it.
Now that you know about the rule of thirds, go try it out. Once you've got the hang of it you can start breaking it.
