poem vs tanka

poem and tanka both are nouns.

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

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

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

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