Usually when you send
email to a
valid address, your message
is
appended to a file (or stuck into a
database) that
represents your friend's
mailbox. Now if someone sets up
an autoreply, the mail goes into a
program instead.
(It may or may not go into a mailbox as well.)
The autoreply program picks your address out of the
header, and sends you a canned reply. Some autoreplies
are smarter than others, and they might scan your email
(or at least the Subject line) and find a canned response
chosen specially for you!
Useful when you're on vacation, so co-workers and friends
know why you're not replying to your mail.
Bad when you're on a mailing list! Because if the mailing list program is not very bright, it won't filter out
your autoreplies, and you can have an infinite email loop
that constantly doubles (because you reply to your autoreplies). Most email lists nowadays filter out
autoreplies, or have Reply-To headers set to the original
sender (so your autoreply doesn't go to the list). Also,
a good autoreply program will not reply when its own
work comes back to them.