Sterilization is a surgical procedure, making it impossible for the man or woman to have children.
It’s a way to achieve permanent infertility. You should only have this done if you are absolutely sure you don’t want (more) children.
When either the man or the women is sterilized the couple doesn’t have to worry anymore about unwanted pregnancy.
Sometimes people regret the operation, for instance when they start a new relationship or after the death of a child. They might ask for an operation to restore fertility. But sterilization is not easily undone (the success rate is about 50%). So you should never be sterilized if you are not absolutely sure that this is the right decision. The younger you are, the greater the chance you’ll regret it later on.
Sterilization is very effective, but not 100% reliable. Very rarely pregnancy occurs. Often this is because the woman was already pregnant before sterilization or the couple start sex too soon after a vasectomy.
Male sterilization is called vasectomy and female sterilization tubal ligation (or tying the tubes). More information on sterilization can be found on those pages.