Simply put, ekphrasis is writing about the visual arts. The writing could be poetry or prose, a play or a novel, as long as the writing attempts to describe a piece of visual art—a painting, sculpture, tapestry, whatever—in some way or another.
John Keats's "Ode on a Grecian Urn" is a famous example of ekphrasis, as is Homer's description of the Shield of Achilles in The Iliad.
