National Guard vs ARNG

National Guard and ARNG both are nouns.

Word	Noun	Adjective	Verb	Adverb
National Guard	Yes	No	No	No
ARNG	Yes	No	No	No

As nouns, ARNG is a hyponym of National Guard; that is, ARNG is a word with a more specific, narrower meaning than National Guard:

National Guard: United States military reserves recruited by the states and equipped by the federal government; subject to call by either
ARNG: a civilian reserve component of the United States Army comprised of guardsmen who serve during overseas peacekeeping missions and during local emergencies

Other hyponyms of National Guard include Air National Guard, ANG, Army National Guard.

National Guard (noun)	ARNG (noun)
United States military reserves recruited by the states and equipped by the federal government; subject to call by either	a civilian reserve component of the United States Army comprised of guardsmen who serve during overseas peacekeeping missions and during local emergencies