A sig block is a small piece of text identifying a poster or mailer. It goes at the end -- thus it's a bit like a signature.
Sig blocks have a long history. Originally used to give bang paths(!), the Internet, new routing protocols and especially the DNS MX record meant they were no longer necessary. But by now they had moved into new territory, containing canned words of wisdom from the author. One's sig was an important part of one's online persona.
Naturally, as the September that never ended grew on, sig blocks grew longer. After all, a mere 576 lines may be enough to describe anyone else, but surely ariels cannot fit into such a limiting format! The only things more annoying than these monster sigs were the flame wars that they provoked regularly. Eventually Kibo's signature evolved, once again proving Kibo's status as the (pantless) Gaudi of the Internet.
Nowadays sigs look back on their golden age. Many people don't have sigs. Alternatively, Internet "aware" companies shove their canned sigs down people's emails. These sigs usually look like this:
This email is the exclusive property of MegaCorp. International, a subsidiary of the
E2WDC. It is intended only for its intended recipient. If you have received this email
by mistake or because our managers are as incompetent at emailing as they are at everything
else, please destroy all copies, shred your
computer, return all copies of the ema
il to MegaCorp. International, and report for termination at ou
r earliest convenience.
only in
HTML, and maybe a bit of
Flash.
The famous RFC that never was, Son of 1036, tried to specify a standard format for sigs. It says to start the sig with a line containing exactly 2 dashes and a space ("-- "). It also recommends that mailing and posting software (e.g. email programs and newsreaders) keep sig blocks below 5 lines. Unlike regular RFCs, which aren't followed, this one is usually ignored. Except by the aforementioned corporations, which, it seems, do it on purpose.