As a photographer, light means everything to me. The quality of light is especially important. In this image, it is the lack of light that tells the story. Personally, for this image I am attracted to the soft feel of the horse's body and the cowgirl's careful step.
Silhouettes are fairly easy images to photograph -- once you know how! The secret is to position the subject against an open sky when the light is fading. Make sure they are as parallel as they can get to you and at the crest of the rise (so feet don't blend into the darkness). You should be able to see as much of the lines as possible ... the more lines and shapes, the more interesting the silhouette becomes. It takes some practice, but it's worth it!
Post production is also essential in creating a good silhouette. In Lightroom, you will want to pull your black point and shadows until the details fall away and you are left with a silhouette. Then add in vibrance, saturation and clarity as needed. Finally, use a graduated filter to attain a beautiful color to your image. Lavender and purple are some of my favorite colors (and you'll see that a lot in my work!).