From Dictionary.com:
Main Entry: baby shower
Part of Speech: n
Definition: a party given for a pregnant woman, to which guests bring presents for the baby
Example: Having a baby shower arranged by a family-member or relative is seen as bad form and self serving by many traditionalists.
Etymology: 1902
Wikipedia (as faulty of a source as any) also mentions that baby showers have traditionally been for the first born child rather than subsequent children. Baby showers are to impart wisdom to a new mother and "shower" them with gifts that a new mother and new baby might need. More recently, baby showers can occur for every baby born.
I used to love, love baby showers. I loved the games, and the fun gathering, and the excitement of a new baby. I still do, however, my love for them has waned recently. Mostly because some of the showers I have been invited to recently have been for 2nd or 3rd kids - when I've already been to a shower for the 1st kid. Didn't they get "showered" with enough gifts from their first kid? I was there. I saw how much they raked in. I don't know. Maybe not. And, on top of that, these showers seem to be given by the pregnant person's mom or mother-in-law. Is that tacky or "self-serving"? Also, an e-vite on facebook, to me, does not replace a good ol' fashioned snail mail invitation.