A term used to denote mail sent via the traditional postal service, usually to contrast that process to e-mail.
Mail sent through regular postal service: Acrobat has the potential to pay for itself rather quickly by eliminating the need to send documents by courier or even regular mail (snail mail, as it is charmingly called by computer aficionados)
[1980s+ Computer; referring to the slowness of the snail]
(Or "snailmail", "smail" from "US Mail" via "USnail"; "paper mail"). Bits of dead tree sent via the postal service as opposed to electronic mail. One's postal address is, correspondingly, a "snail (mail) address". There have even been parody USnail posters and stamps made.
The variant "paper-net" is a hackish way of referring to the postal service, comparing it to a very slow, low-reliability network. Sig blocks sometimes include a "Paper-Net:" header just before the sender's postal address; common variants of this are "Papernet" and "P-Net". Note that the standard netiquette guidelines discourage this practice as a waste of bandwidth, since netters are quite unlikely to casually use postal addresses and if they really wanted your snail mail address they could always ask for it by e-mail.
Compare voice-net, sneakernet, P-mail.