The term "composition" denotes the overall arrangement of a work of art. In order to describe the composition of a painting (for instance), one must identify the parts the painting is composed of (e.g. figures, buildings, natural features), and then describe how those parts are arranged.
Pyramidal vs. Dynamic Composition
Two of the most prominent approaches to visual composition are the pyramidal composition of High Renaissance painting and the dynamic composition of Baroque painting (see Western Aesthetics). Pyramidal composition focuses strongly on unity, while dynamic composition is primarily concerned with contrast.
The figures in Leonardo's Virgin of the Rocks are arranged such that the "pinnacle" of the painting (Mary's head) is flanked by figures to either side and to the fore. The arrangement of these figures is characterized by symmetry and balance. A sense of stability is achieved, and the eye is naturally drawn to a single point (Mary's face). In terms of colour, the patches of bluish sky in the upper part of the painting are balanced with the bluish robes in the lower part; other colours are likewise balanced.
Ruben's Raising of the Cross, on the other hand, features diagonal configurations of figures. The overall arrangement is characterized by asymmetry and imbalance. A strong sense of dynamism (a sense that the painting is "in motion") results as the eye is drawn restlessly along each line. Note also the unbalanced use of colour; for instance, while the left and centre panel feature a dark background, the right panel is set against a light sky.
Unity vs. Contrast
A sense of unity can be achieved through repetition of parts, as well as balanced arrangement of parts. Likewise, a sense of contrast can be achieved by using sharply different parts, and by arranging those parts in an unbalanced manner.
Arguably, all great art must strike a balance between unity and contrast. Unity is like an engine (stable and carefully structured), while contrast is like the burning fuel inside the engine (active and uncontrolled). An engine without fuel won't run; likewise, a work of art with "excessive unity" is mechanical and dull. Conversely, fuel without an engine will simply burn out of control; likewise, a work of art with "excessive contrast" is messy and directionless.