Forum spambots surf the web, looking for guestbooks, wikis, blogs, forums and any other web forms to submit spam links to the web forms it finds. These spambots often use OCR technology to bypass CAPTCHAs present. Some spam messages are targeted towards readers and can involve techniques of target marketing or even phishing, making it hard to tell real posts from the bot generated ones. Not all of the spam posts are meant for the readers; some spam messages are simply hyperlinks intended to boost search engine ranking.
This category of spambots has gained considerable notoriety since November 2006, with the introduction of XRumer, a forum and wiki spambot which can often bypass many of the safeguards administrators use to reduce the amount of spam posted.
The easiest way to prevent spambots from posting on forums, wiki, guestbook, etc. is to enable e-mail activation by installing a mail server on the host (eg: Sendmail, Postfix, Exim.), since most spambot scripts use fake or randomly generated names on real e-mail providers, the e-mails will mostly never be successfully routed to them.