Birth control is used when couples (and they usually are couples) do something that stops them from having children, because they don't want children, because they don't want children yet, because they don't want more children. Birth control is necessary since the world is overpopulated. Religious people (namely, fundies) believe that no human life should be taken away, and that humans need to overpopulate the world because they are superior. Isn't it gross cruelty to create more human beings if you are convinced that the overwhelming majority will end up in Hell?
Some examples are the birth-control pill, and condoms. Abortion is unfortunately sometimes used as birth control but most people think it's better to prevent a pregnancy starting than to do abortion later. When religious people discourage use of contraception or prevent young people finding out how to use contraception this can and does lead to abortion later.