Sunni vs Muslim

Sunni and Muslim both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sunni	Yes	No	No	No
Muslim	Yes	Yes	No	No

As nouns, Muslim is a hypernym of Sunni; that is, Muslim is a word with a broader meaning than Sunni:

Sunni: a member of the branch of Islam that accepts the first four caliphs as rightful successors to Muhammad
Muslim: a believer in or follower of Islam

Other hypernyms of Sunni include Moslem.

Sunni (noun)	Muslim (noun)
one of the two main branches of orthodox Islam	a believer in or follower of Islam
a member of the branch of Islam that accepts the first four caliphs as rightful successors to Muhammad

Sunni (adjective)	Muslim (adjective)
	of or relating to or supporting Islamism