robber vs mugger

robber and mugger both are nouns.

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

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

robber: a thief who steals from someone by threatening violence
mugger: a robber who takes property by threatening or performing violence on the person who is robbed (usually on the street)

Other hyponyms of robber include bank robber.

robber (noun)	mugger (noun)
a thief who steals from someone by threatening violence	a robber who takes property by threatening or performing violence on the person who is robbed (usually on the street)