The intended purpose of the pill is to work in three ways:
1) To prevent ovulation.
2) Thicken the mucous around the cervix, thus preventing sperm from reaching the uterus.
3) Alter the uterine lining to make implantation of the egg difficult.
Just go to any site which explains how the pill works, and they will state this. Here is one example: http://www.webmd.com/sex/birth-control/birth-control-pill
Note, however, that it is impossible to say how frequent the 3rd-stage "defense" against life occurs in real life. It could be 1%, 10% or 50% - I know of no research that conclusively shows how frequent each "defense" occurs.