Composition is the placement elements in an image, the centre of interest is not normally the centre of the frame. The viewer will then tend to linger over points of interest that his eyes lead to. This is referred to as the "rule of thirds" a principle taught in graphic design and photography and is based on the theory that the eye goes naturally to a point about two thirds up from the bottom of the image. By visually dividing the image into thirds either horizontally or vertically or you achieve asymmetric balance which is naturally pleasing to the eye.
Nikon D300, Nikkor 70-200mm 2.8 @ ISO 200, f2.8, 78mm, 1/640 sec
Some principles of composition are shape and proportion, repetition (pattern), perspective, consistency (among elements), balance (of elements), orientation, space, colour, contrast, lighting and field of view or path of the the viewer's eye when they observe the image. Sometimes you need something in the image that leads you into the picture or gives you a sense of where the photographer was, to his 'perspective'. For example, in the second image from the same place but with a wider frame of view.