belle vs young lady

belle and young lady both are nouns.

Word	Noun	Adjective	Verb	Adverb
belle	Yes	No	No	No
young lady	Yes	No	No	No

As nouns, young lady is a hypernym of belle; that is, young lady is a word with a broader meaning than belle:

belle: a young woman who is the most charming and beautiful of several rivals
young lady: a young female

Other hypernyms of belle include fille, girl, miss, missy, young woman.

belle (noun)	young lady (noun)
a young woman who is the most charming and beautiful of several rivals	a young female