1990s vs decade

1990s and decade both are nouns.

Word	Noun	Adjective	Verb	Adverb
1990s	Yes	No	No	No
decade	Yes	No	No	No

As nouns, decade is a hypernym of 1990s; that is, decade is a word with a broader meaning than 1990s:

Other hypernyms of 1990s include decennary, decennium.

1990s (noun)	decade (noun)
the decade from 1990 to 1999	the cardinal number that is the sum of nine and one; the base of the decimal system
	a period of 10 years