literature vs piece of writing

literature and piece of writing both are nouns.

Word	Noun	Adjective	Verb	Adverb
literature	Yes	No	No	No
piece of writing	Yes	No	No	No

As nouns, piece of writing is a hypernym of literature; that is, piece of writing is a word with a broader meaning than literature:

literature: creative writing of recognized artistic value
piece of writing: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of literature include writing, written material.

As nouns, piece of writing is a hypernym of literature; that is, piece of writing is a word with a broader meaning than literature:

literature: published writings in a particular style on a particular subject
piece of writing: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of literature include writing, written material.

literature (noun)	piece of writing (noun)
the profession or art of a writer	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)
the humanistic study of a body of literature
creative writing of recognized artistic value
published writings in a particular style on a particular subject