twenty-one vs cards

twenty-one and cards both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-one	Yes	Yes	No	No
cards	Yes	No	No	No

As nouns, cards is a hypernym of twenty-one; that is, cards is a word with a broader meaning than twenty-one:

twenty-one: a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21
cards: a game played with playing cards

Other hypernyms of twenty-one include card game.

twenty-one (noun)	cards (noun)
a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21	a game played with playing cards
the cardinal number that is the sum of twenty and one

twenty-one (adjective)	cards (adjective)
being one more than twenty