head word vs content word

head word and content word both are nouns.

Word	Noun	Adjective	Verb	Adverb
head word	Yes	No	No	No
content word	Yes	No	No	No

As nouns, content word is a hypernym of head word; that is, content word is a word with a broader meaning than head word:

Other hypernyms of head word include open-class word.

head word (noun)	content word (noun)
(grammar) the word in a grammatical constituent that plays the same grammatical role as the whole constituent	a word to which an independent meaning can be assigned
a content word that can be qualified by a modifier