profession vs line of work

profession and line of work both are nouns.

Word	Noun	Adjective	Verb	Adverb
profession	Yes	No	No	No
line of work	Yes	No	No	No

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

profession: an occupation requiring special education (especially in the liberal arts or sciences)
line of work: the principal activity in your life that you do to earn money

Other hypernyms of profession include business, job, line, occupation.

profession (noun)	line of work (noun)
an occupation requiring special education (especially in the liberal arts or sciences)	the principal activity in your life that you do to earn money
an open avowal (true or false) of some belief or opinion
affirmation of acceptance of some religion or faith
the body of people in a learned occupation