nigga vs Negro

nigga and Negro both are nouns.

Word	Noun	Adjective	Verb	Adverb
nigga	Yes	No	No	No
Negro	Yes	No	No	No

As nouns, Negro is a hypernym of nigga; that is, Negro is a word with a broader meaning than nigga:

nigga: (ethnic slur) extremely offensive name for a Black person
Negro: a person with African ancestry, "Negro" and "Negroid" are archaic and pejorative today

Other hypernyms of nigga include African-American, Black, Negroid, Black person, blackamoor.

nigga (noun)	Negro (noun)
(ethnic slur) extremely offensive name for a Black person	a person with African ancestry, "Negro" and "Negroid" are archaic and pejorative today