extortion vs blackmail

extortion and blackmail both are nouns.

Word	Noun	Adjective	Verb	Adverb
extortion	Yes	No	No	No
blackmail	Yes	No	Yes	No

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

extortion: the felonious act of extorting money (as by threats of violence)
blackmail: extortion of money by threats to divulge discrediting information

Other hyponyms of extortion include protection, tribute, shakedown.

extortion (noun)	blackmail (noun)
the felonious act of extorting money (as by threats of violence)	extortion of money by threats to divulge discrediting information
unjust exaction (as by the misuse of authority)
an exorbitant charge

extortion (verb)	blackmail (verb)
	obtain through threats
	exert pressure on someone through threats