Hi,
It depends on the school. I believe most if not all do snail mail or e-mail. As far as notice most schools give you a few weeks notice. The Ohio State Univeristy is an exception to this ( at least it was for me) I got an e-mail on a Friday that I had an interview the following Wednsday! So not much notice on that one--I don't know if they had that short notice for everyone or if I got unlucky. I applied to nine schools--of the four I interviewed at (tufts, UPenn,Purdue, and ohio) all but one were by snail mail. The earliest interview I had was UPenn in early january. Cornell says they will e-mail you their decision but if you are an alternate they mail a package to you--that was confusing!
Good luck and try not to get too stressed out about the wait.