casework vs social service

casework and social service both are nouns.

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

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

casework: close sociological study of a maladjusted person or family for diagnosis and treatment
social service: an organized activity to improve the condition of disadvantaged people in society

Other hypernyms of casework include welfare work.

casework (noun)	social service (noun)
close sociological study of a maladjusted person or family for diagnosis and treatment	an organized activity to improve the condition of disadvantaged people in society