exertion vs difficulty

exertion and difficulty both are nouns.

Word	Noun	Adjective	Verb	Adverb
exertion	Yes	No	No	No
difficulty	Yes	No	No	No

As nouns, difficulty is a hyponym of exertion; that is, difficulty is a word with a more specific, narrower meaning than exertion:

exertion (noun)	difficulty (noun)
use of physical or mental energy; hard work	an effort that is inconvenient
	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result