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. The process of fusion also improves the search results.
Metasearch engines create what is known as a virtual database. They do not compile a physical database or catalogue of the web. Instead, they take a user's request, pass it to several other heterogeneous databases and then compile the results in a homogeneous manner based on a specific algorithm.
No two metasearch engines are alike. Some search only the most popular search engines while others also search lesser-known engines, newsgroups, and other databases. They also differ in how the results are presented and the quantity of engines that are used. Some will list results according to search engine or database. Others return results according to relevance, often concealing which search engine returned which results. This benefits the user by eliminating duplicate hits and grouping the most relevant ones at the top of the list.
Search engines frequently have different ways they expect requests submitted. For example, some search engines allow the usage of the word "AND" while others require "+" or only a space to combine words. The better metasearch engines try to synthesize requests appropriately when submitting them. 
Architecture of a metasearch engine
Examples of Metasearch Engine
· Info.com – It provides results from leading search engines and pay-per-click directories, including Google, Yahoo!, Bing.com, Ask, LookSmart, About and Open Directory.
· HotBot.com – In the 1990s, it was one of the most popular search engines on the World Wide Web.
· Yippy.com – Offers clusters of results.
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