Search engines often use the number of backlinks that a website has as one of the most important factors for determining that website's search engine ranking. Websites often employ various techniques (called search engine optimization) to increase the number of backlinks pointing to their website.

There are several factors that determine the value of a backlink. Backlinks from authoritative sites on a given topic are highly valuable. If both sites have content geared toward the keyword topic, the backlink is considered relevant and believed to be have strong influence on the search engine rankings of the webpage granted the backlink. A backlink represents a favorable 'editorial vote' for the receiving webpage from another granting webpage. Another important factor is the anchor text of the backlink. Anchor text is the descriptive labeling of the hyperlink as it appears on a webpage. Search engine bots (i.e., spiders, crawlers, etc.) examine the anchor text to evaluate how relevant it is to the content on a webpage. Anchor text and webpage content congruency are highly weighted in search engine results page (SERP) rankings of your webpage with respect to any given keyword query by a search engine user.

Most commercial search engines provide a mechanism to determine the number of backlinks they have recorded to a particular web page. For example, Google can be searched using link: wikipedia.org (or link: en.wikipedia.org) to find the number of pages on the Web pointing to http://wikipedia.org/.

Yahoo!’s Site Explorer is a method of obtaining the number of backlinks on a site.

On the World Wide Web, a link farm is any group of web sites that all hyperlink to every other site in the group. Although some link farms can be created by hand, most are created through automated programs and services. A link farm is a form of spamming the index of a search engine (sometimes called spamdexing or spamexing). Other link exchange systems are designed to allow individual websites to selectively exchange links with other relevant websites and are not considered a form of spamdexing.

Link farms were developed by search engine optimizers in 1999 to take advantage of the Inktomi search engine's dependence upon link popularity. Although link popularity is used by some search engines to help establish a ranking order for search results, the Inktomi engine at the time maintained two indexes. Search results were produced from the primary index which was limited to approximately 100 million listings. Pages with few inbound links continually fell out of the Inktomi index on a monthly basis.

Inktomi was targeted for manipulation through link farms because it was then used by several independent but popular search engines, such as HotBot. Yahoo!, then the most popular search service, also used Inktomi results to supplement its directory search feature. The link farms helped stabilize listings primarily for online business Web sites that had few natural links from larger, more stable sites in the Inktomi index.

Link farm exchanges were at first handled on an informal basis, but several service companies were founded to provide automated registration, categorization, and link page updates to member Web sites.

When the Google search engine became popular, search engine optimizers learned that Google's ranking algorithm depended in part on a link weighting scheme called PageRank. Rather than simply count all inbound links equally, the PageRank algorithm determines that some links may be more valuable than others, and therefore assigns them more weight than others. Link farming was adapted to help increase the PageRank of member pages.

However, even the link farms became susceptible to manipulation by unscrupulous webmasters who joined the services, received inbound linkage, and then found ways to hide their outbound links or to avoid posting any links on their sites at all. Link farm managers had to implement quality controls and monitor member compliance with their rules to ensure fairness.

Alternative link farm products emerged, particularly link-finding software that identified potential reciprocal link partners, sent them template-based emails offering to exchange links, and create directory-like link pages for Web sites hoping to build their link popularity and PageRank.

Search engines countered the link farm movement by identifying specific attributes associated with link farm pages and filtering those pages from indexing and search results. In some cases, entire domains were removed from the search engine indexes in order to prevent them from influencing search results.

Anchor text is weighted (ranked) highly in search engine algorithms, because the linked text is usually relevant to the landing page. The objective of search engines is to provide highly relevant search results; this is where anchor text helps, as the tendency is, more often than not, to hyperlink words relevant to the landing page.

Webmasters may use anchor text to procure high results in search engine results pages. Google's Webmaster Tools facilitate this optimization by letting website owners view the most common words in anchor text linking to their site.

In the past, Google bombing has been possible through anchor text manipulation; however, in January, 2007, Google announced it had updated its algorithm to minimize the impact of Google bombs.