A link farm is a group of websites that all link to each other. This particular form of inbound linking is considered an unethical search engine optimization practice because it never offers the end-user relevant, valuable information related to his search query.
This Black Hat seo device is a form of ‘spamdexing’ and is designed to boost search engine ranking by tricking the search engines. In order to accurately index and retrieve documents, a search engine must group things according to relevancy and the perceived popularity of the content as it relates to a specific search word or phrase. Inbound links, also called backlinks, pointing from one external site back to another website are treated by the search engines as a sort of popularity “vote.” The website that the external link is pointing to is considered “popular” because it offers relevant information that is visited often by outside sources. See ‘Inbound Links’ and ‘PageRank’ for more information.