smell vs sniff

smell and sniff both are nouns.

Word	Noun	Adjective	Verb	Adverb
smell	Yes	No	Yes	No
sniff	Yes	No	Yes	No

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

Other hyponyms of smell include snuff.

As verbs, sniff is a hyponym of smell; that is, sniff is a word with a more specific, narrower meaning than smell:

Other hyponyms of smell include whiff, nose, scent, wind, snuff, snuffle, get a noseful, get a whiff.

smell (noun)	sniff (noun)
the act of perceiving the odor of something	sensing an odor by inhaling through the nose
any property detected by the olfactory system
the faculty that enables us to distinguish scents
the sensation that results when olfactory receptors in the nose are stimulated by particular chemicals in gaseous form
the general atmosphere of a place or situation and the effect that it has on people

smell (verb)	sniff (verb)
become aware of not through the senses but instinctively	inhale audibly through the nose
emit an odor	perceive by inhaling through the nose
inhale the odor of; perceive by the olfactory sense
smell bad
have an element suggestive (of something)