Link spamming is the concept of building (buying) a large number of inbound links to a Web site/page with the sole purpose of boosting that page's search engine ranking. Link spamming is inspired by search engines' reliance on link analysis as critical element in their ranking calculations for Web content. Search engines are acutely aware of the popularity of link spamming and thus subject indexed content to thorough link analysis before deciding on the importance and relevance of a given page.
When performing link analysis, search engines tend to weigh the quality of inbound links over the quantity. Thus, a single inbound link from a high-ranking, preferably topically-related Web page will have a bigger impact on a Web page's search engine ranking than a large number of back links from low-ranking, seemingly unrelated sites. Many of the Internet's free-for-all link exchange venues and link farms allow users to buy/obtain hundreds, even thousands of back links in one sweeping move. Usually, there is no guarantee that the links will originate from sites that have any relation to the site they will be pointing to. Bulk link exchange/link acquisition thus may be considered link spamming.
Search engine Google recognizes this problem and have thus made link-acquisition a factor in its review of indexed content. In essence: Because link analysis is the single most important Google ranking factor, the search engine is particularly vigilant about combating link spamming. This means that Web pages that seem to gain thousands of back links overnight immediately will raise Google's suspicion — unless there is an obvious reason why those links appeared. Google prefers that pages build links at a steady pace.
Both Google and Yahoo! rely on so-called "sandboxing" concepts when reviewing newly indexed pages. Web pages that reside in the sandbox are deliberately kept out of the top rankings for as much as several months. The search engines do this as a countermeasure against search engine spamming. Sandboxing pages allow search engines to take the necessary time to carefully review new pages to assess their quality and importance. As well, sandboxing allows the search engines to prevent pages that rely on blatant link spamming from tainting the search results.