How does the birth control pill work?

I have a friend is who very confused about how the birth control pill works, so I just want to confirm.He thinks it works by killing off all fertilized eggs. In other words, he believes pregnancy will always happen, but it will kill the pregnancy. I told him that it is meant to work in the way that it prevents ovulation, therefore preventing an egg from even being released, but he still claims he is right.Who is right, and can anyone provide links?If it is still confusing, he believes that if a woman has sex on the pill, she will become pregnant and the pregnancy will either implant and be killed by the pill, or the fertilized egg will be killed before it can implant in the uterus. Either way, he believes the pill is meant to cause a miscarriage or an abortion.Just to clarify, he is not my boyfriend. I am married. This guy is a friend of ours. ------