Variety is the use of differences and change to increase the visual interest of a work of art.
Variety works through juxtaposition and contrast. When an artist places different visual elements next to one another, he/she is using variety. Straight lines next to curvy lines add variety. Organic shapes among geometric shapes add variety. Bright colors next to dull colors add variety.
If an artist uses variety to draw the viewers attention to a specific area in a composition then variety morphs into emphasis, also a principle of art. Principles of art bleed into one another. They overlap.
Below is a painting with lots of variation in color, shape and texture, yet not so much that it loses any sense of harmony.
Below is a painting with comparatively less variance. The result is a much calmer painting.
A lot of variety can make an artwork look busy or overwhelming. When paired with unity, variety offers the viewer points of interest.
In this example of variety in art, Kandinsky uses a variety of lines, shapes, values, and colors.