lettuce vs herb

lettuce and herb both are nouns.

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

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

lettuce: any of various plants of the genus Lactuca
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 lettuce include herbaceous plant.

lettuce (noun)	herb (noun)
leaves of any of various plants of Lactuca sativa	aromatic potherb used in cookery for its savory qualities
any of various plants of the genus Lactuca	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests
informal terms for money