tae kwon do vs martial art

tae kwon do and martial art both are nouns.

Word	Noun	Adjective	Verb	Adverb
tae kwon do	Yes	No	No	No
martial art	Yes	No	No	No

As nouns, martial art is a hypernym of tae kwon do; that is, martial art is a word with a broader meaning than tae kwon do:

tae kwon do: a Korean martial art similar to karate
martial art: any of several Oriental arts of weaponless self-defense; usually practiced as a sport

tae kwon do (noun)	martial art (noun)
a Korean martial art similar to karate	any of several Oriental arts of weaponless self-defense; usually practiced as a sport