There are many elements to a piece of art; speaking specifically of visual art, for example, we may discuss the medium, the technique, the genre or historical precedents, etc. When we discuss the “content” of a work of (visual) art, we are generally concentrating on its relation to reality, its “story” or its “representation of reality.” The “content” of The Last Supper is not its technique or history or place in the Renaissance, but the “story” it is depicting—the story of Christ and the Apostles at the Last Supper. We may include discussions of its subtleties – the placement of John the Beloved, the possibility of Mary Magdalene in the picture, the emblem of the bag of gold, etc.—these are all subjects of “content.” When we discuss whether Michelangelo or Da Vinci painted it, or the techniques of mural painting, or the politics of the Catholic Church, we are not speaking of “content.” Again, when we speak of the Mona Lisa’s content, we are concerning ourselves with the person depicted, the background landscape, the smile, but when we discuss chiaroscuro or the way a painting gains in value, we are no longer discussing “content.” There was (is?) a painting in San Francisco which was entitled something like “Study No. 3”; when patrons insisted on a more descriptive title, the artist, addressing the “content,” renamed it “The Unnatural Battle of the Four Primal Elements”. In much modern art, the forms in a painting do not submit to a discussion of “content” since the artist’s intentions are not to tell a story but to present forms and/or color only.
In the other arts, for example drama, the “content” is the story being told, and the era depicted, rather than the style of dialogue or age of original production. The “content” of Hedda Gabler is the changes in Hedda’s life after her father’s death and how those changes move the story forward. Slightly more abstractly, we can say the “content” is the struggle of women in a man’s world.