Depending on the method used, emergency contraceptives can work in several ways.
First, it works by preventing or delaying the release of an egg from the woman's ovaries. No egg, no pregnancy. If an egg IS released, emergency contraception may slow down the egg and/or sperm in the fallopian tubes, possibly preventing fertilization. Finally, the hormones can thin the lining of the uterus, keeping a fertilized egg from implanting.
It is important to note, that emergency contraception is NOT an abortion pill. Most types of emergency contraception will have no effect if a woman is already pregnant.