Link scheme
A link scheme is any arrangement of links intended to manipulate rankings, which Google's policies explicitly prohibit.
Link scheme is Google's umbrella term for manipulative linking practices. Its policy names specific examples: buying or selling links that pass ranking signals, excessive reciprocal link exchanges, large-scale article marketing or guest-post campaigns built only for links, automated link-building programs, and requiring a link as a condition of a partnership or contract. Links inside ads or sponsorships are fine only when marked with rel sponsored or nofollow link.
It matters because participating in a link scheme is the single most common cause of a links-related manual action, and it is exactly the behavior google penguin and SpamBrain are designed to catch. Both ends are at risk: the site receiving manipulative links and the site selling them can be penalized.
The practical line is intent and disclosure. A guest post can be a link scheme or a legitimate contribution depending on whether it exists to pass link equity or to reach an audience. A paid placement is fine if it is disclosed and tagged so it passes no ranking signal. When money or a quid pro quo changes hands for a followed link, you are almost certainly inside scheme territory, so tag it correctly or do not do it.