I’m slightly embarrassed to say I’ve never consciously realized that there is a difference between “afterword” and “afterward” (or “afterwards”). I wonder how many times I’ve confused these words (probably not terribly many, since, not having written
many any books anything containing epilogues, I’ve had little occasion to throw “afterword” around).
afterword, n. A passage added at the end of a book, etc., as an epilogue or the like.
afterward, adv. Of time: In time following, subsequently.
afterwards, adv. At a later time, subsequently.
[from the Oxford English Dictionary]