Image via WikipediaAccording to The WWW Robot Page, these agents normally start with a historical list of links, such as server lists, and lists of the most popular or best sites, and follow the links on these pages to find more links to add to the database.[1]
A meta-search engine is a search tool that sends user requests to several other search engines and/or databases and aggregates the results into a single list or displays them according to their source. Meta search engines enable users to enter search criteria once and access several search engines simultaneously. Meta search engines operate on the premise that the Web is too large for any one search engine to index it all and that more comprehensive search results can be obtained by combining the results from several search engines. This also may save the user from having to use multiple search engines separately.[2]
A subject directory is a catalog of sites collected and organized by humans. Subject directories are often called subject "trees" because they start with a few main categories and then branch out into subcategories, topics, and subtopics.
Because directories cover only a small fraction of the pages available on the Web, they are most effective for finding general information on popular or scholarly subjects.[3]
[1] How search engines work?
www.webreference.com/content/search
[2] Meta search engine
www.ask.com/wiki/Metasearch_engine
[3] Subject Directories
www.learnwebskills.com/search/subject.html
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