human beings vs human being

human beings and human being both are nouns.

Word	Noun	Adjective	Verb	Adverb
human beings	Yes	No	No	No
human being	Yes	No	No	No

As nouns, human being is a hypernym of human beings; that is, human being is a word with a broader meaning than human beings:

human beings: all of the living human inhabitants of the earth
human being: any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage

Other hypernyms of human beings include group, grouping, homo, human, man.

human beings (noun)	human being (noun)
all of the living human inhabitants of the earth	any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage