In terms of a painting, composition refers 1. color choice 2. imagery 3. organization of the painting's various images on the canvas 4. style

3. organization of the painting's various images on the canvas

Explanation:

The composition is the disposition of the elements on the canvas. That's the essence of the art piece usually, that's where the artist express his/her ideas and choses the way to present it to the world.  

Color choice is also important, but not as much, unless it's a totally abstract painting.

It's the same thing in photography and in many other art forms.