Composition
The spatial and proportional organization of elements into a coherent whole.
In practice
Composition is the overall organization of elements in a given space: where they sit, how large they are relative to each other, and how they relate. It is the decision behind everything else on the page. A concert poster might place one huge photo, the band name along the top edge and the dates clustered small at the bottom right, guiding the eye from image to name to details.
Often confused with Layout
Composition is the broader art of organizing elements into a whole, in any medium; layout is the practical arrangement of content within a specific page or screen.
Try this
Cut out five paper shapes, arrange them three different ways on a sheet, photograph each and choose the strongest.