A sig link is a link that is present in a users signature. This signature can be either in forums, chat rooms, or in emails. Sig links consist of anchor text, URL and sometimes a short description for the link.
Why Use A Sig Link
Sig links are used because each time a user posts on a forum or sends an email a link to the site that webmaster is promoting will show up for the reader. Sig links will also add to the number of backlinks a site has each time the sig link appears on a public forum. Sig links are therefore used to both generate traffic by getting readers to click on the sig link and to generate backlinks.
Sig Link Limitations
Each forum will have its own set of rules regarding sig links. There will likely be a character limit for the sig, a limit to the total number of links allowed and rules as to which types of sites can be linked to in signatures. Email sig links generally have no limitations.