The Search Engine Index
Search engines maintain a vast database of information about sites they have crawled with a spider called an index. This index contains the page URL, keywords found on the page, titles, links, and other meta information.
This index is used to drive the actual search engine itself. Using the index, search engines such as Google can determine which pages are relevant to a given query. Additional factors such as links back to a site can help influence how highly a page may rank for a query as well.