tanka vs poem

tanka and poem both are nouns.

Word	Noun	Adjective	Verb	Adverb
tanka	Yes	No	No	No
poem	Yes	No	No	No

As nouns, poem is a hypernym of tanka; that is, poem is a word with a broader meaning than tanka:

tanka: a form of Japanese poetry; the 1st and 3rd lines have five syllables and the 2nd, 4th, and 5th have seven syllables
poem: a composition written in metrical feet forming rhythmical lines

Other hypernyms of tanka include verse form.

tanka (noun)	poem (noun)
a Tibetan religious painting on fabric	a composition written in metrical feet forming rhythmical lines
a form of Japanese poetry; the 1st and 3rd lines have five syllables and the 2nd, 4th, and 5th have seven syllables