Link spam
Links created to manipulate rankings: bought links, link exchanges, private blog networks and similar schemes.
Google's link spam policy covers links bought or sold for ranking, excessive link exchanges, automated link creation and links from networks built for the purpose. Paid and sponsored links are allowed when marked with rel="sponsored" or rel="nofollow".
Google's current approach is mostly to neutralise spammy links — ignore them — rather than penalise the site they point to, though manual actions remain possible.