How the pill works
The pill, and other horomonal contraceptives such as the patch and vaginal ring, create a perfect, yet artificial cycle. All of these methods contain artificial hormones (ethinyl estradiol to mimic estrogen, and/or progestin to mimic progesterone) that prevent your body from ovulating. This is accomplished because these fake hormones trick your ovaries into believing that ovulation has already occurred. So when you're on the pill, you don't truly ovulate; ovulation is simulated. This is now the case with even progestogen only pills (also know as the mini-pill), older versions of which would allow ovulation most of the time.
If your body does manage to ovulate on the pill, you are still unlikely to become pregnant. This is because the pill also creates an inhospitable environment for sperm survival (by thickening cervical fluid) and egg implantation (by thinning the uterine lining).
Because of the birth control pill's last pregnancy preventation mechanism (shrinking of the endometrium) it has become a controversial topic for those who believe that human life begins when the egg is fertilized.