friend vs fellow

friend and fellow both are nouns.

Word	Noun	Adjective	Verb	Adverb
friend	Yes	No	No	No
fellow	Yes	No	No	No

As nouns, fellow is a hyponym of friend; that is, fellow is a word with a more specific, narrower meaning than friend:

friend (noun)	fellow (noun)
a person you know well and regard with affection and trust	a boy or man
a person with whom you are acquainted	a man who is the lover of a man or woman
an associate who provides cooperation or assistance	a friend who is frequently in the company of another
a person who backs a politician or a team etc.	a person who is member of one's class or profession
	an informal form of address for a man
	a member of a learned society
	one of a pair