The results are usually ranked according to how closely they match the keywords, as defined by the search engine's set of variable criteria. Behind the Scenes An important point regarding search engines is that the user's search is a search of the engine's index and not of the web itself. The web search, performed by the spider, occurs earlier. The amount of the web crawled by the spider determines the size of the engine's library of web documents. No search engine is able to cover the entire web.
Pages appear and disappear hourly.
The size of the web is mammoth, and it continues to grow at an exponential rate. Search engines differ as to what percentage of the web they cover, their techniques used in obtaining coverage, and their selectivity in eliminating junk. How frequently the engine's spider crawls the web will also influence the user's choice of search tool. This is of significant importance in an age where old information is quickly superseded by the new. All search engines support single- word queries.