COMPOSITION: the way the components parts (the formal elements - line, shape, form, texture, space, value, color) of a work of art are arranged in the picture plane or given space.

How the formal elements are arranged not only determines the strength of a composition in a given picture plane, but also are essential in conveying, supporting, and communicating the content or the idea and meaning.

PLEASE NOTE: Compositional arrangements are also biased to our cultures, in that in Western culture, we read from left to right and from the top down. In other cultures, this is not the case. So often in our Western approach to looking at images, we tend to 'read' images in this same fashion, from left to right and then top-down.

Rembrandt van Rijn, The Anatomy Lesson of Dr. Tulp, 1632, Oil on Canvas

For example, in the oil painting, The Anatomy Lesson of Dr. Tulp, by Rembrandt van Rijn (shown above), he has intensionally staged the composition of this painting to evoke and communicate a particular mood. If we are to focus on a few elements:

VALUE, the overall value of this piece has low contrast - mostly dark with a few moments of light, which creates a sense of mystery. The area with the most contrast is the lightest value shape of the corpse against the very dark attire of the figures surrounding it, which encourages the viewer to look here first and then visually move elsewhere.
SHAPE - most the collars of the mens' outfits appear as triangles, which acts as a device to intensionally move the viewer's eyes throughout the piece.
The viewer's eye movement throughout the piece is also reinforced by the implied LINEs created through the gaze of the different figures in the image.

In this particular painting and paintings created prior to and during this time, the artist didn't always have the luxury of having all of the subjects/figures in the room at the same time. It is through an intensional layout or composition that Rembrandt created, which allowed him to fill in the portrait and hands of the sitter at different times.

Composition provides a sense of intentionality on behalf of the artist.