The third type of search engine is the metasearch engine; examples include Metacrawler and Dogpile (links open new windows). Metasearch engines allow the searcher to examine a number of different search engines, both indices and directories, at the same time. Since no search engine can contain information on every possible Website, the metasearch engine provides the advantage of searching a number of different resources, each potentially containing different links, at the same time. The disadvantage: limited ability to tailor the search according to the strengths of a particular database.
Search engines provide a powerful mechanism with which to access the multitude of resources that are contained within the Web. When doing a comprehensive search, e-searchers should select several different search engines in which to run their search, and should become familiar with the search instructions that are included on each page. Given the vastness of the information that they are examining, no search engine can contain everything, and no search is actually done in "real time." Search engines periodically examine each of the sites in their database, using "crawlers" (or "worms" or "spiders" or "bots"), which attempt to keep up with the changing landscape of the Web. This is, of course, an impossible task, and results in Web searches that may display links to sites that no longer exist or which have changed so drastically that they no longer contain the information which is being sought.