I found this on Wikipedia: https://en.wikipedia.org/wiki/Spambot
Forum spambots surf the web, looking for guestbooks, wikis, blogs, forums, and other types of web forms that it can then use to submit bogus content. These often use OCR technology to bypass CAPTCHAs. 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. Other spam messages are not meant to be read by humans, but are instead posted to increase the number of hyperlinks to a particular web site, to boost its search engine ranking.