shop clerk vs shop girl

shop clerk and shop girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
shop clerk	Yes	No	No	No
shop girl	Yes	No	No	No

As nouns, shop girl is a hyponym of shop clerk; that is, shop girl is a word with a more specific, narrower meaning than shop clerk:

Other hyponyms of shop clerk include shop boy.

shop clerk (noun)	shop girl (noun)
a salesperson in a store	a young female shop assistant