volunteer vs worker

volunteer and worker both are nouns.

Word	Noun	Adjective	Verb	Adverb
volunteer	Yes	Yes	Yes	No
worker	Yes	No	No	No

As nouns, worker is a hypernym of volunteer; that is, worker is a word with a broader meaning than volunteer:

volunteer (noun)	worker (noun)
a person who performs voluntary work	sterile member of a colony of social insects that forages for food and cares for the larvae
(military) a person who freely enlists for service	a person who works at a specific occupation
	a member of the working class (not necessarily employed)
	a person who acts and gets things done

volunteer (adjective)	worker (adjective)
without payment