What exactly is the "morning-after pill"?

Now, let me make it clear that I don't intend on using this, as I'm not ually active and do not plan to be for a very long time. But I've heard about the "morning-after pill" and don't know what it does and am confused. Does it prevent the sperm from reaching the egg, or does it kill the very young fetus? Or do something entirely different? I just want to know out of curiousity. Thanks.