letterhead vs letter paper

letterhead and letter paper both are nouns.

Word	Noun	Adjective	Verb	Adverb
letterhead	Yes	No	No	No
letter paper	Yes	No	No	No

As nouns, letter paper is a hypernym of letterhead; that is, letter paper is a word with a broader meaning than letterhead:

letterhead: a sheet of stationery with name and address of the organization printed at the top
letter paper: paper cut to an appropriate size for writing letters; usually with matching envelopes

Other hypernyms of letterhead include stationery.

letterhead (noun)	letter paper (noun)
a sheet of stationery with name and address of the organization printed at the top	paper cut to an appropriate size for writing letters; usually with matching envelopes
	writing paper for use in writing correspondence