taekwondo vs martial art

taekwondo and martial art both are nouns.

Word	Noun	Adjective	Verb	Adverb
taekwondo	Yes	No	No	No
martial art	Yes	No	No	No

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

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

taekwondo (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