Forum spam

Forum spam is the posting of advertisements, abusive, or unwanted messages on Internet forums. It is generally posted by automated spambots. The link sits quietly in the signature field, where it is more likely to be harvested by search engine spiders than discovered by forum administrators and moderators.

Most forum spam consists of links to external sites, with the dual goals of increasing search engine visibility in highly competitive areas such as weight loss, pharmaceuticals, gambling, pornography, real estate or loans, and generating more traffic for these commercial websites. 

Some of these links contain code to track the spambot's identity if a sale goes through, when the spammer behind the spambot works on commission. Alternately, the spam links are posted in the user's signature, in which case the spambot will never post.

Spam posts may contain anything from a single link, to dozens of links. Text content is minimal, usually innocuous and unrelated to the forum's topic, or in a very old thread that is revived by the spammer solely for the purpose of spamming links.