A metasearch 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.
Search engines are of three types. Directory Based, Meta Based and Crawler based. Now a days we use crawler based engines(Google, Bing and Yahoo). DMOZ is a directory based search engine. Dogpile, Brainboost are some examples of meta based search engines. Meta based engines fetch data for user's query from various data bases and not only from world wide web.