The first approach is using a so-called search engine. A search engine allows the user to enter keywords that run against a database. Single search engines search a single database. Meta or multi-thread search engines allow the user to search multiple databases simultaneously, via a single interface. While they do not offer the same level of control over the search interface and search logic as do individual search engines, most of the multi-threaded engines are very fast.
Subject guides The second approach is to use so-called subject guides.
Subject guides are hierarchically organized indexes of subject categories that allow you to browse through lists of Web sites by subject. They are compiled and maintained by humans and many include a search engine for searching their own database. Specialized subject guides compiled by a subject specialist concentrate on specific subjects. They are more likely to contain important resources in the area of expertise than a general subject guide. Since these subject guides are compiled and maintained by humans, you'll often find some kind of site quality rating.