art vs illustration

art and illustration both are nouns.

Word	Noun	Adjective	Verb	Adverb
art	Yes	No	No	No
illustration	Yes	No	No	No

As nouns, illustration is a hyponym of art; that is, illustration is a word with a more specific, narrower meaning than art:

Other hyponyms of art include drawing.

art (noun)	illustration (noun)
the creation of beautiful or significant things	a visual representation (a picture or diagram) that is used to make some subject more pleasing or easier to understand
the products of human creativity; works of art collectively	an item of information that is typical of a class or group
a superior skill that you can learn by study and practice and observation	artwork that helps make something clear or attractive
photographs or other visual representations in a printed publication	showing by example