You search the index of the descriptions. The index is what you are searching when you enter terms in a search engine. Google, Northernlight, and AltaVista are examples of search engines.
Most directories also maintain a search box for searching accross the database.
No one search engine seaches the entire web. Try your search in different ones. HotBot has a drop-down box. Dogpile Metacrawler Internet guides or Virtual libraries select Web sites and put them in subject categories. As with any other source, Web sites require proper citation.