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Illustration art, aka "commercial" art is used to embellish, clarify, or decorate something. It can range from a simple black-and-white cartoon to a full-color billboard and beyond. Illustrators in the Golden Age were required to draw and paint expertly and fast. They might be required -- in a few days -- to provide artwork for a battle scene, a love story, a historical drama, a glimpse of the future, and a beautiful American girl. The best of the best commanded top dollar and were as famous as their Hollywood and Broadway pals.
Illustration art is also something which has been readily embraced by Joe and Jane America who ordered the illustrator's calendars, asked the magazines for prints, and made scrapbooks of a favorite artist's work.
On the other hand, Illustration Art is something that is almost universally rejected by today's art elites, the people who think that if it's obtuse, ugly, or out there, it's art. If not, not.