It's difficult to finger the essential difference between illustration and works of fine art. But to state plainly, I think illustration is an aspect of imagination that subordinates itself to a story or concept in order to realize a visual equivalent of the verbal idea. Illustration is description executed in a graphic rather than literary mode.[1] Whereas artists and illustrators commonly work from the same stock stories and ideas, artists use imagination in its purer sense; - not to describe visually what can be articulated verbally (to think through words) but to compose with a more essentially pictorial logic (thinking through images).
As the same sets of skills are necessary to both fields (anatomy, perspective, projection, colour harmony, etc.) I can only conclude that the difference is in the function of the imaginative faculty. Works of fine art, like illustration, often take narrative as their subject matter, but perhaps they work out their compositions in a less verbally oriented or descriptive process of the imagination. To borrow an example from the movies, Alfred Hitchcock claimed that he tried to make films that communicated through images. The distinction is in the mode of storytelling: a purely imaginative sequence of dream-like images (dream-like because dreams tap into a primal form of communication that relies more on images than language).
There is no ground for supposing that illustrations are inartistic, kitsch, or inferior in any sense of technique, ingenuity, visual interest, etc.
regards,
Iian
End notes:
[1] Illustration can legitimately be used in a linguistic sense: "he illustrated his point with well chosen examples", "can you illustrate for me what you're talking about", etc., where the sense of illustration is description. Of course, description itself can be graphic (description as detail in a drawing), verbal, or gestural ("his arm described an arc in the air").
