obstacle vs snag

obstacle and snag both are nouns.

Word	Noun	Adjective	Verb	Adverb
obstacle	Yes	No	No	No
snag	Yes	No	Yes	No

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

obstacle: something immaterial that stands in the way and must be circumvented or surmounted
snag: an unforeseen obstacle

Other hyponyms of obstacle include barrier, roadblock, hang-up, hitch, rub, hurdle, stymie, stymy.

obstacle (noun)	snag (noun)
an obstruction that stands in the way (and must be removed or surmounted or circumvented)	an unforeseen obstacle
something immaterial that stands in the way and must be circumvented or surmounted	an opening made forcibly as by pulling apart
	a dead tree that is still standing, usually in an undisturbed forest
	a sharp protuberance

obstacle (verb)	snag (verb)
	catch on a snag
	hew jaggedly
	get by acting quickly and smartly