ancestor vs forefather

ancestor and forefather both are nouns.

Word	Noun	Adjective	Verb	Adverb
ancestor	Yes	No	No	No
forefather	Yes	No	No	No

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

ancestor: someone from whom you are descended (but usually more remote than a grandparent)
forefather: the founder of a family

Other hyponyms of ancestor include ancestress, forbear, forebear, father, sire, foremother, primogenitor, progenitor.

ancestor (noun)	forefather (noun)
someone from whom you are descended (but usually more remote than a grandparent)	the founder of a family
	person from an earlier time who contributed to the tradition shared by some group