Permanent birth control means it cannot be reversed if you change your mind. Even though with modern technology complicated reversal processes can possibly reverse certain procedures, a person contemplating having one of these procedures should go into it thinking it could never be reversed. If you are not 100% sure you never want to have more children no matter what happens, don't do it.
For men, vasectomy is the permanent method, and for women it is tubal ligation, and now there is a new minimally invasive procedure where the fallopian tubes are blocked through a vaginal approach with no incision, called Essure.