viola vs herb

viola and herb both are nouns.

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

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

viola: any of the numerous plants of the genus Viola
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 viola include herbaceous plant.

viola (noun)	herb (noun)
any of the numerous plants of the genus Viola	aromatic potherb used in cookery for its savory qualities
a bowed stringed instrument slightly larger than a violin, tuned a fifth lower	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests