Comment spam refers to the unsolicited or automated posting of irrelevant or promotional comments in online platforms, such as blogs, forums, or social media, with the intention of generating links or driving traffic to external websites.
Spammers hope to piggyback on the targeted domain's stronger search engine presence. This concept is based on the fact that backlinks from quality websites are a ranking factor.
However, many sites moderate comments and disallow those with irrelevant text and links. Furthermore, many commenting plugins automatically turn published links into “nofollow” links, which convey no SEO value.
Link sellers often promise large numbers of links and include comment spam in their efforts.