work of art vs piece

work of art and piece both are nouns.

Word	Noun	Adjective	Verb	Adverb
work of art	Yes	No	No	No
piece	Yes	No	Yes	No

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

work of art: art that is a product of one of the fine arts (especially a painting or sculpture of artistic merit)
piece: a work of art of some artistic value

Other hyponyms of work of art include magnum opus, art object, objet d'art, pastiche, period piece, warhorse.

work of art (noun)	piece (noun)
art that is a product of one of the fine arts (especially a painting or sculpture of artistic merit)	a separate part of a whole
	game equipment consisting of an object used in playing certain board games
	a portable gun
	a work of art of some artistic value
	an item that is an instance of some type
	a distance
	a musical work that has been created
	an artistic or literary composition
	an instance of some kind
	a serving that has been cut from a larger portion
	a portion of a natural object
	a share of something
	a period of indeterminate length (usually short) marked by some action or condition

work of art (verb)	piece (verb)
	repair by adding pieces
	eat intermittently; take small bites of
	join during spinning
	to join or unite the pieces of
	create by putting components or members together