Whenever a keyword is searched for in a search engine, then the search engines calculate the relevance of a site to that keyword. The number of back links is calculated. Some search engines, especially the reputed ones filter the sites which do not have quality back links. They consider only the number of quality inbound links to that site. So the number of back links to a website does not matter and does not increase its ranking. The number of quality back links to a website is of utmost importance. The quality of a back link is determined by its content. Many people buy backlinks which are sold in large quantities. These back links contain content that are not related to the website it points to. When inbound links to your site come from other sites, they are considered as votes to your website. If those sites have content related to your site, then these inbound links are considered more relevant to your site. Hence one must buy back links which contain related content and are from a reputable source. If inbound links have unrelated content, then they are considered less relevant. For example, if you host a website which deals about guitar music, then you must back links related to guitar and music. Instead if you buy back links related to mountaineering, then these are considered irrelevant and such back links will seriously bring down the ranking of your website.