Composition is the placement or arrangement of the elements or your subject(s) within the scene you are photographing. In other words the placement of, which is composition. The term Rule of Thirds is about composition and the placement of your subject. This guideline, or Rule of Thirds, generally suggests that you place your subject somewhere outside of the center of the scene.
For example; if you are on vacation and you are about to take a photo of a bird in a tree, or a person on the beach. Deciding where to place the main subject, such as off to one side, (in the middle), or the top half of your scene is considered a composing or composition in preparation for taking the photograph.