The only difference between ancestors and forefathers is that our forefathers are multiple persons and communal, regional, or national predecessors (from whom we inherit language, customs.
Our ancestors are the individual and personal family tree predecessors (from whom we inherit DNA internal to ourselves).
However ancestors are referred to as forefathers as well so the ancestors and forefathers are essentially the same thing but there's a slight difference but they are the same people basically.
Our ancestors are parents or (recursively) the parent sof an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth).
As for a forefather which is basically the same as ancestors are a member of the past generations of one's family or people; an ancestor.