What made Google become very successful was finding ways to index credible websites while eliminating search results that were more of interested in driving traffic to them than relevancy of the content. Google's method for establishing trust was to count the quantity and quality of links to any given website from other pages on the internet, and to use a formula for doing this automatically. This formula is known in the search engine world as the "Google Algorithm."
Search engine algorithm is a complex set of rules that defines how any given page on the internet relates to any other page. The Google Algorithm has over 200 different factors that help determine a web page's relevance for any particular keyword. Some parts of the algorithm are closely guarded as a trade secret while others are widely well known. Even though search engine algorithms can be very complex, it is still possible to configure your website so that it gets properly classified by algorithms from major search engines.