On the World Wide Web, a link farm is any group of web sites that all hyperlink to every other site in the group. In graph theoretic terms, a link farm is a clique. Although some link farms can be created by hand, most are created through automated programs and services.
Link farms were originated in response to Google’s ranking algorithm, PageRank. PageRank thinks of links as votes, where a page linking to another page is casting a vote. Therefore, pages with the most links were deemed valuable. Link farms were created to help increase PageRank, however, in recent times, Google and other search engines have adapted their algorithms to prevent link farms from appearing at the top of search engine results pages (SERPs).
Web sites that have created link farms can be penalized by Google and other search engines.
Link farming is considered a “black hat” method of search engine optimization (SEO) because the content on the web sites linking to each other is often not related, not created by a human (it can be created using programs or services) and is often of a very low quality. Search engines can detect link farms easily and they shouldn’t be seen as a legitimate long-term search optimization strategy.
explains Link Farming
Although relevant inbound links are an important component of website SEO, this should not be confused with listing a site on a link farm, which does nothing more than link to other sites in a group.
Search engines such as Google view link farming as a form of spam. Google and others have implemented procedures to punish sites that gain inbound links through farms, rather than organically or through link campaigns with other high-quality sites.
Service providers who promise to improve a website’s search engine rank may use link farming. In this situation, the provider simply posts links en masse on other unrelated websites, some of which are designed only for this purpose.
Search engines used to be very vulnerable to link farming because they primarily operated on the reasoning that a page has greater authority if a large number of other sites link to it. Now, link farming often fails because search engines have evolved to rank results based on what will be most useful for the user. This means that a link from a site with trusted content or similar content to the page to which it is linking is worth more than random links from many sites.