roe deer vs deer

roe deer and deer both are nouns.

Word	Noun	Adjective	Verb	Adverb
roe deer	Yes	No	No	No
deer	Yes	No	No	No

As nouns, deer is a hypernym of roe deer; that is, deer is a word with a broader meaning than roe deer:

roe deer: small graceful deer of Eurasian woodlands having small forked antlers
deer: distinguished from Bovidae by the male's having solid deciduous antlers

Other hypernyms of roe deer include cervid.

roe deer (noun)	deer (noun)
small graceful deer of Eurasian woodlands having small forked antlers	distinguished from Bovidae by the male's having solid deciduous antlers