The lines that can be seen in a scene can be powerful compositional elements that make a photograph more dynamic as well lead the viewer into a photo.
Lines can be horizontal, vertical, diagonal or circular. Lines exist in numerous scenes we see. Often we do not think about them because the three dimensional nature of our sight tends to obscure them.
What is obscure in a three dimensional scene is often more noticeable in a two dimensional scene. To get a two dimensional view of any scene you want to photograph just close one eye while looking.
Before you frame a photograph in your camera, think about what lines can be seen and how they could add something to the photograph.
The gallery has a few examples of using lines as part of the composition.