Directories can find information quickly because the listed sites have been screened by the directory service staff and assigned to the specific category. Therefore, you're not likely to find someone's home page that just happens to mention basketball. Directories are hierarchically organized indexes of subject categories that allow the web searcher to browse through lists of web sites by subject in search of relevant information. As a result, directory searches tend to produce more relevant listings, and since they usually return links to the top level of a web site instead of an individual page they lend themselves better to searching for information about a general subject, rather than a specific piece of information. These spider programs search the web collecting information about new or updated sites to put into the database.
Search engines allow users to enter keywords that are searched against a database.
Based on a combination of criteria the search engine retrieves web documents from its database that match the user entered keywords. AltaVista, Excite, Infoseek, and Lycos are all popular search engines. The advantage of search engines is that they're nearly comprehensive in that they include hundreds sites in the results listed. Each search engine uses a proprietary algorithm to determine which of the millions of pages in its database relate to the topic entered in the search. Search engines are useful when looking for a specific topic that probably doesn't appear in a directory.