This refers to the addition of search results to the rankings as a result of payment rather than through the natural discovery process that search engines follow. There is some blurring of the distinction between these listings and pay per click advertising. The most obvious difference is that these are sold without specified guaranteed wording and unlike PPC ads appear within the organic search results instead of in a separate, clearly marked advertising block.
It’s definitely worth noting that Google, the most dominant search engine, does not allow users to buy their way into the organic listings. The only way to be listed through payment is in the clear advertising blocks through their search advertising programmes. This avoids the situation other search engines face where they can be accused of returning results that are of financial benefit to the search provider but add less value to the end user.
Nonetheless it is popular where available and delivers many benefits such as more regular crawling and immediate listing which gives new businesses a faster road to exposure in the organic results. It also allows for quicker feedback to be received on any changes made to content which in itself is a useful SEO tool.