tobacco vs herb

tobacco and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
tobacco	Yes	No	No	No
herb	Yes	No	No	No

As nouns, herb is a hypernym of tobacco; that is, herb is a word with a broader meaning than tobacco:

tobacco: aromatic annual or perennial herbs and shrubs
herb: a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests

Other hypernyms of tobacco include herbaceous plant.

tobacco (noun)	herb (noun)
leaves of the tobacco plant dried and prepared for smoking or ingestion	aromatic potherb used in cookery for its savory qualities
aromatic annual or perennial herbs and shrubs	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests