human face vs smiler

human face and smiler both are nouns.

Word	Noun	Adjective	Verb	Adverb
human face	Yes	No	No	No
smiler	Yes	No	No	No

As nouns, smiler is a hyponym of human face; that is, smiler is a word with a more specific, narrower meaning than human face:

human face: the front of the human head from the forehead to the chin and ear to ear
smiler: the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)

Other hyponyms of human face include countenance, kisser, mug, phiz, physiognomy, visage.

human face (noun)	smiler (noun)
the front of the human head from the forehead to the chin and ear to ear	the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)
	a person who smiles