's apple is a little bulging spot in human male
's neck. It's really larynx
that took a growing spurt during your puberty
, but note that it's not male-specific organ, only organ that's larger on males by average, and therefore it's possible women could have something that looks like "adam's apple" and still have been born with, khm, ordinary plumbing, so to speak.
It's not known why it's larger with males (or so it seems to me), but it's one of the causes for characteristically lower voice males have.
I've also heard (Helsingin Sanomat- Sunnuntailiite- kysy) that the reason it's more visible on males is because females have more fat in their necks, burying the adam's apple. Beats me.
Its name originates from biblical tale, in which it is told that when Adam tried to eat the apple that snake offered him, it got stuck in his throat and staid there as punishment for his sin. Although why it was inherited to his children, beats me.
Finally, according to Devil's Dictionary, it is "A protuberance in the throat of man, thoughtfully provided by Nature to keep the rope in place.".