One of the major rules of composition in photography is called the Rule of Thirds. The rule of thirds states that before you capture the subject, you should consider how the composition will look when split into thirds vertically and horizontally. It has been noted in studies that the point of intersection for these thirds is typically the first place the eye travels when viewing an image. So separating the image into thirds creates a natural compositional element.
As with most rules of composition and photography, they are meant to be broken. However, before you break these rules you should have a clear understanding of why you are breaking the rules and how that will ultimately impact the viewers experience with the photograph.
Take this example of two owls. The rules of thirds lines have already been applied so that you can see exactly how it works. As you can see, both owls sit on opposite ends of the vertical thirds with space between them and body and head separated by horizontal thirds. Not only does this composition allow you to see, in full, both owls, but it also helps to give a clear sense of depth using the shallow depth of field technique.
Now imagine that a photo was taken of the owl on the left, except that owl was in the middle of the frame. It makes you realize just how much impact the rule of thirds can make