robber vs thief

robber and thief both are nouns.

Word	Noun	Adjective	Verb	Adverb
robber	Yes	No	No	No
thief	Yes	No	No	No

As nouns, thief is a hypernym of robber; that is, thief is a word with a broader meaning than robber:

robber: a thief who steals from someone by threatening violence
thief: a criminal who takes property belonging to someone else with the intention of keeping it or selling it

Other hypernyms of robber include stealer.

robber (noun)	thief (noun)
a thief who steals from someone by threatening violence	a criminal who takes property belonging to someone else with the intention of keeping it or selling it