today vs nowadays

today and nowadays both are nouns.

Word	Noun	Adjective	Verb	Adverb
today	Yes	No	No	Yes
nowadays	Yes	No	No	Yes

As nouns, nowadays is a hypernym of today; that is, nowadays is a word with a broader meaning than today:

today: the present time or age
nowadays: the period of time that is happening now; any continuous stretch of time including the moment of speech

Other hypernyms of today include present.

As adverbs, today and nowadays are synonyms defined as:

Other synonyms of today include now.

today (noun)	nowadays (noun)
the present time or age	the period of time that is happening now; any continuous stretch of time including the moment of speech
the day that includes the present moment (as opposed to yesterday or tomorrow)

today (adverb)	nowadays (adverb)
on this day as distinct from yesterday or tomorrow	in these times
in these times