Link spam
Link spam is creating links to or from a site mainly to manipulate search rankings. Google's policies used to call it "link schemes".
Updated , every quotation and figure read in its source that day.
Google's spam policies define link spam as "the practice of creating links to or from a site primarily for the purpose of manipulating search rankings". The examples they list: buying or selling links for ranking purposes (money, goods or free products exchanged for links), excessive link exchanges, automated link building, requiring a link as part of a contract or terms of service, text ads and advertorials whose links pass ranking credit, links with optimized anchor text in articles, guest posts or press releases distributed on other sites, low-quality directory or bookmark links, widely distributed footer or template links, and forum comments with optimized links.
Paid links are not banned as such. Google writes that buying and selling links "is a normal part of the economy of the web for advertising and sponsorship purposes", and that such links are fine as long as they carry rel="nofollow" or rel="sponsored". What breaks the rules is a paid link that passes ranking credit. A private blog network built to sell links is the clearest case.
The usual outcome is that the links stop counting: Google's systems, including SpamBrain, neutralize manipulative links rather than count them. Large, deliberate patterns can also lead to a manual action for unnatural links. Spam is about pattern and intent rather than one link: a single paid link without rel="sponsored" is unlikely to sink a site, while identical anchors, the same networks and an unnatural link velocity make a profile easy to flag.