Your 2D-design textbook* identifies three basic components to a work of art:
"A subject is a person, a thing, or an idea… In abstract or semi-abstract works, the subject may be somewhat perceivable, but in non-objective works, the subject is the idea behind the form of the work, and it communicates only with those who can read the language of form… "(p.10)
The term form is used in various ways when referring to art objects. Speaking of a piece of sculpture, one may refer to the individual forms that together make up the piece or one may speak of the sculpture's form, meaning its total appearance…" (p.12)
The emotional or intellectual message of an artwork is its content, a statement expression, or mood read into the work by its observer… For many people content is confined to familiar associations, usually feelings aroused by known objects or ideas… "(p.12)

*Ocvirk, Otto G., Stinson, Robert E., et al. Art Fundamentals: Theory and Practice. New York: McGraw Hill, 1998.