http://en.wikipedia.org/wiki/Castrato
Its basically getting your male genital cut off. So you basically seem like a girl without one.
But i saw a BBC documentary about it. And on part two. They say, if a boy was castrated before reaching puberty. Then he wouldnt develop things like beard etc. Is that true?
http://youtu.be/uWRurzEPAcA
1:04 - 2:02