Using lines to draw your eye.

An old barn in Central Alberta - the lines make a difference.

Photography can be described as the art of capturing light.  It is more than just the rendering of photons reflected from an object to produce a two-dimensional display though, it is about the relationships of things in that image.  A good example of this is seen in the above photo.

The barn itself is a wonderful subject.  Full of character, its weathered features sculpted over the years bear testament to the service it has provided.  Old, but still useful, and literally full of life, it renders not only function but beauty to the landscape on which it sits.  The surrounding vegetation seems to understand this; the shadows from the trees and even the limbs themselves are drawn to it.  

Lines are part of the world we live in.  They are the natural way that physical boundaries and ephemeral shadows are perceived.  They can be more than that though if given the opportunity.  Lines can be used to draw attention to important aspects of your creation.  We have an innate tendency to follow lines; our eyes and visual processing center enhance them and are guided by them.  Notice how the shadows of the trees draw your gaze towards the barn itself.  The branches intruding into the scene are not a mere coincidence but rather artfully placed to direct your attention.  The truck itself even adds to the effect.  The contrast of the old and the new, the hint of function and value, and the lines from its trim pay further homage to the edifice of farm living.

Use lines to help emphasize your subject by drawing further attention to it.  They should not overwhelm the photo or take away from what you are trying to achieve.  Rather, they should act as a subtle reminder of what is important.  There are relationships between all parts of the image; the lines act to enhance them.  Pointing, leading, and themselves adding texture to the scene, lines are a critical component in any photograph.