At the end of most articles is a small blurb called a person's
signature. In Unix this file is named .signature in the
person's login directory---it will vary for other operating systems.
It exists to provide information about how to get in touch with the
person posting the article, including their email address, phone
number, address, or where they're located. Even so, signatures have
become the graffiti of computers. People put song lyrics, pictures,
philosophical quotes, even advertisements in their ".sigs".
(Note, however, that advertising in your signature will more often
than not get you flamed until you take it out.)
Four lines will suffice---more is just extra garbage for Usenet sites
to carry along with your article, which is supposed to be the intended
focus of the reader. Netiquette dictates limiting oneself to this
"quota" of four---some people make signatures that are ten lines or
even more, including elaborate ASCII drawings of their hand-written
signature or faces or even the space shuttle. This is not
cute, and will bother people to no end.
Similarly, it's not necessary to include your signature---if you
forget to append it to an article, don't worry about it. The
article's just as good as it ever would be, and contains everything
you should want to say. Don't re-post the article just to include the
signature.