These professionals classify useful web sites into a hierarchical, subject-based structure. Yahoo is the most widely known and most widely used directory service. Yahoo and other directories are very useful when looking for information in clear categories, for example listings on federal and state agencies. Essentially you just drill down on the categories until you find a screen of useful sites within a single category. The second type of search services is search engines.
Search engines behave differently than directories in that they index words in web site documents.
Excite, AltaVista and Infoseek are some of the more popular search engines. To find web site documents in a search engine you ask a question or submit a query. The search engine then matches your query words against the indexed words it has in its databases and presents a listing of possible documents. Typically, search engines are the most useful when looking for more difficult topic areas or for those topics that do not fit into the clear categories used by directories. In some shape or form our readers have sighted that their biggest frustration lies in their inability to find the information they are looking for.