Taglines are small (usually a single line of less than 80 characters) quotes placed at the end of emails or newsposts. Sometimes they can be found in other media such as books and webpages, or used as quit messages in chat channels. Because brevity is considered so desirable the attribution of the quote is often omitted and put somewhere seperately (e.g. in the headers of the mail or post, or inside a <abbr> tag).
I use the retag.pl script from my codepile to add my signature and a random tag at the bottom of my mail and newsgroup posts. I have the script bound to shift-S in vim so I can cycle through a few random ones.