Actually, I suspect what you mean is not a caesarean section. Caesarean section is when the mother goes under an fully-fledged operation and the baby is brought out just below the navel.
What I supect you mean is, when under *normal birth*, they cut part of the woman to make it easier for the baby to come out. This is fairly common, and the mother does not feel it because she is in so much pain anyway.
It is called an episiotomy.
If you are really that interested, you can get a full description here:
http://www.babycenter.co.uk/refcap/165.html#0