Typology of Web Search Engines
Yahoo: yahoo always uses the directory base search service and search for directories
Google: Google uses a search engine, then web index. Google uses spider or robot in web index.
Meta Engine: A search engine that queries other search engines and then combines the results that are received from all. In effect, the user is not using just one search engine but a combination of many search engines at once to optimize Web searching. For example, Dogpile is a Metasearch engine.
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. Metasearch engines enable users to enter search criteria once and access several search engines simultaneously. Metasearch 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.