girlfriend vs lover

girlfriend and lover both are nouns.

Word	Noun	Adjective	Verb	Adverb
girlfriend	Yes	No	No	No
lover	Yes	No	No	No

As nouns, lover is a hypernym of girlfriend; that is, lover is a word with a broader meaning than girlfriend:

Other hypernyms of girlfriend include adult female, woman.

girlfriend (noun)	lover (noun)
a girl or young woman with whom a man is romantically involved	a person who loves someone or is loved by someone
any female friend	an ardent follower and admirer
	a significant other to whom you are not related by marriage