Link spam
Link spam is any link created primarily to manipulate search rankings rather than to genuinely help users, and it violates Google's spam policies.
Google defines link spam as links built mainly to influence rankings instead of serving readers. This covers a wide range of tactics: buying or selling links that pass link equity, excessive link exchanges, large-scale automated link creation, low-quality directory or bookmark links, and links embedded in spun or thin guest content. The common thread is intent. If a link exists to game the algorithm rather than to inform or reference, Google treats it as spam.
Why it matters is straightforward. Google's private blog network detection and its SpamBrain system increasingly neutralize manipulative links rather than count them, so spammy links often do nothing at best. At worst they trigger algorithmic suppression or a manual action, dragging down rankings you earned legitimately. The economics rarely work out: cheap links at scale are exactly what Google is best at ignoring.
A practical nuance: spam is about pattern and intent, not a single link. One paid link with a missing rel="sponsored" tag is unlikely to sink a site, but a consistent footprint of identical anchors, the same link networks, and unnatural link velocity creates a profile that is easy to flag. Build for relevance and editorial merit, and the spam question mostly takes care of itself.