a set of protocols or rules that search engines use to rank the listings contained within their index in response to a particular search query. It is a sophisticated program of rules that the search engine developers create to determine and rank the degree of relevance for data.

The search engine algorithms contain complex components that determine how a web page is ranked.  Components of the algorithm include on-page factors as well as linking factors from other websites. The original paper describing Google's algorithm was written by Sergey Brin amd Lawrence Page and is still available on Stanford's website.