Factual Question on Oral Contraception

After re-reading some of these posts on contraception and pharmacists and looking over some of the left-wing blogs' takes on this question, I am a little bit confused about the distinction between abortifacients and oral contraception.  My understanding is that oral contraceptives are permissible for therapeutic purposes, even though they can prevent implantation of a fertilized ovum.  (Ironically, oral contraceptives are sometimes used as part of fertility treatments (to sort of re-zero the hormonal clock).)  Some of the things I've read suggest that Plan B works in the same way.  What exactly is meant by an abortifacient?  Does the hierarchy consider normal oral contraceptives to be such?  (Is the only difference between oral contraceptives and so-called "emergency contraception" the intent of the user?)  I would have thought that abortifacient means something that directly kills an embryo.  But maybe there's something about the hierarchy's teachings on oral contraceptives that I'm misunderstanding.  Can someone point me towards a good (and recent) scholarly analysis of the question?