There's a lot of misconceptions about what the pill does, and even Aksannyi doesn't have it completely right.
Basically, the pill causes a change of hormones that essentially mimics the effects of the progesterone drop that causes periods. This effectively prevents pregnancies in a variety of ways:
1) Preventing ovulation (although this is irrelevant if an egg is already present)
2) Preventing fertilization (although this is irrelevant if an egg is present and already fertilized)
3) Preventing implantation of a fertilized egg in the uterine lining (although this is irrelevant if a fertilized egg has already implanted)
Source:
http://www.webmd.com/women/guide/plan-b
The only reason it's technically not an abortion pill is because
it doesn't cause a miscarriage if a fertilized egg has already implanted when the pill is taken. However, there is very little real difference between a very early miscarriage (AKA spontaneous abortion), and a normal period that happens to flush out a fertilized egg before it had an opportunity to implant. The latter happens to tens (maybe even hundreds) of thousands of women each month, and most of them don't even realize it happened.
I hate my job for making me learn this much about women's health.