The number of backlinks is an indication of the popularity or importance of that website. Backlinks are important for SEO because some search engines like Google, give more credit to websites that have a large number of quality backlinks, and consider those websites more relevant than others in their results pages for a search query. Therefore, when search engines calculate the relevance of a site to a keyword, they not only consider the number of backlinks to that site but also their quality. In order to determine the quality, a search engine considers the content of the sites. When backlinks to your site come from other sites, and those sites have content related to your site, these backlinks are considered more relevant to your site. If backlinks are found on sites with unrelated content, they are considered less relevant. The higher the relevance of backlinks, the greater their quality. For example, if a webmaster has a website about how to rescue orphaned dogs, and received a backlink from another website about dogs, then that would be more relevant in a search engine's assessment than say a link from a site about car racing. Therefore, higher the relevance of the site linking back to your website, the better the quality of the backlink. Search engines want websites to have a level playing field, and look for natural links built slowly over time. While it is fairly easy to modify your webpages to make them more SEO friendly it is a lot harder for you to influence other websites and get them to link to your website. This is the reason search engines regard backlinks as a very important factor. Further, search engine's criteria for quality backlinks has gotten even tougher, thanks to unscrupulous webmasters trying to achieve these backlinks by deceptive or sneaky techniques, such as hidden links, or automatically generated pages whose sole purpose is to provide backlinks to websites. These pages are called link farms, and they are not only disregarded by search engines, but linking to a link farm could get your site banned entirely.