Answers: A spike of a certain hormone call LH causes you to ovulate. Giving the body specific doses of estrogen and/or progesterone found within BC pills prevents the spike of LH, and you do not ovulate your egg.
I guess you are talking more or less the oral contraceptive pill. There are different types of pills, containing different substances. The most commonly used are the combined oestrogen and progestagen pill. Women get different level of hormones during the menstrual cycle which cause the ovum to develop surrounded by the ovary, to be then released during ovulation. The pill act in a style that it inhibits this process and doesn't let ovulation go down. Therefore if there is ovum, pregnancy can not turn out.