I'd have to look it up, don't know off the top of my head, but I'll give a rough explanation.

There are two major varieties of the pill. The one in question works in to ways.
1. It works to prevent the egg from being fertilized.
2. (Backup plan in case it gets fertilized) It does something to the lining of the uterus so that the fertilized egg doesn't set. Basically it rejects the egg with sperm in it. Abortion by some peoples definition.