tuber vs yam

tuber and yam both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuber	Yes	No	No	No
yam	Yes	No	No	No

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

tuber: a fleshy underground stem or root serving for reproductive and food storage
yam: edible tuber of any of several yams

Other hyponyms of tuber include water chestnut, groundnut, potato bean, wild bean, Jerusalem artichoke.

tuber (noun)	yam (noun)
a fleshy underground stem or root serving for reproductive and food storage	sweet potato with deep orange flesh that remains moist when baked
	edible tuberous root of various yam plants of the genus Dioscorea grown in the tropics world-wide for food
	any of a number of tropical vines of the genus Dioscorea many having edible tuberous roots
	edible tuber of any of several yams