alley vs back street

alley and back street both are nouns.

Word	Noun	Adjective	Verb	Adverb
alley	Yes	No	No	No
back street	Yes	No	No	No

As nouns, alley and back street are synonyms defined as:

Other synonyms of alley include alleyway.

alley (noun)	back street (noun)
a narrow street with walls on both sides	a narrow street with walls on both sides
a lane down which a bowling ball is rolled toward pins