Subject directories index Web sites by providing a list of subject categories. Specific topics within each category are arranged hierarchically. Directories include pre-selected sites and have high precision search results. They are a good place to start a search or browse when you don't know specific terminology.
Indices provide lists of gateway sites.
These search engines require the searcher to construct the search by entering search terms into a search form. Many offer simple and advanced searches, with various options for using Boolean operators, truncation, search limitations, ranking and result displays. Query search engines are best for more complex searches, specialized topics, and when comprehensive searches are desired. They are maintained by a variety of sources including academic, professional, and commercial organizations. The following list represents some of the most useful gateways currently available.