Search engine ranking

Overview
Because search results often produce thousands of pages, search engines use ranking systems that attempt to display the most relevant search results at the beginning of the search results. In ranking web pages, search engines follow a certain set of rules. These rules vary from one search engine to another.

To achieve the rankings, search engines look for the location and frequency of keywords and phrases on a web page and, sometimes in the HTML metatags. They check the title field and scan the headers and text near the top of the document. Some of them assess popularity by the number of links that are pointing to the site; the more links, the greater the popularity, i.e., value of the page.

Source
NATO, Intelligence Exploitation of the Internet, at 42.