social work vs social service

social work and social service both are nouns.

Word	Noun	Adjective	Verb	Adverb
social work	Yes	No	No	No
social service	Yes	No	No	No

As nouns, social service is a hypernym of social work; that is, social service is a word with a broader meaning than social work:

social work: any of various services designed to aid the poor and aged and to increase the welfare of children
social service: an organized activity to improve the condition of disadvantaged people in society

Other hypernyms of social work include welfare work.

social work (noun)	social service (noun)
any of various services designed to aid the poor and aged and to increase the welfare of children	an organized activity to improve the condition of disadvantaged people in society