Because there is so much information on the Web, good and bad, finding what you want is not an exact science and can be time consuming. Though many search engines rank material according to their idea of what is relevant, that doesn't mean the material is relevant to want you want or is reliable. The information is only as good as the source. Some sites, says Reuter, protect their proprietary interests by trying to hid their content from search engines.
Search engines use specialized software to create their listings automatically.
The search software looks through the index to find matches to a user's search terms and ranks them in order of perceived relevance. The search service's idea of relevancy may or may not be relevant for your purposes. WWW site creators may submit their sites for inclusion in a subject listing or a directory manager may find them. The search software looks for matches only from the chosen sites. The search software can only operate under the parameters set up by the programmer.