Search engine rankings

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.

source: wikipedia.org