Engine

Engine


A search engine works by looking up information on the internet based on what you ask it to look for. You type words or phrases you want more information about in the search box, click on search, and get the results. 

       A search engine does exactly what its called - it searches certain parts of the internet database based on the words you are searching for information on. For example, if you want football stats, it's not gonna bother looking on Cosmopolitans website, and likewise if you wanted makeup hints, they wouldn't check NFL.com.

1.Directory search engine      Most people are familiar with the concept of search engines these days. In fact, most Internet users turn to search engines on an almost daily basis to help locate information on a specific topic. Internet directories on the other hand are often overlooked, misunderstood and underappreciated by searchers looking for just the right site and by site owners looking to improve their traffic. 

      Often mistaken for search engines, directories are actually collections of human reviewed web sites that have been arranged into topical categories. Popular directories like Yahoo!, Best of the Web and Skaffe can serve as excellent starting points for navigating the Internet. They can also serve as a valuable resource for small business owners looking for ways to drive more traffic to their web sites.

2.General search engine
       General search engines, including web search engines, selection-based search engines, metasearch engines, desktop search tools, and web portals and vertical market websites that have a search facility for online databases.

General:
Ask.com (known as Ask Jeeves in the UK) 
Baidu (Chinese, Japanese) 
Bing (formerly MSN Search and Live Search) 
Blekko 
Duck Duck Go 
Google 
Kosmix 
Sogou (Chinese) 
Yodao (Chinese) 
Yahoo! Search 
Yandex (Russian) 
Yebol 


3.Metasearch search engine      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 term "metasearch" is frequently used to classify a set of commercial search engines, see the list of search engines, but is also used to describe the paradigm of searching multiple data sources in real time. The National Information Standards Organization (NISO) uses the terms Federated Search and Metasearch interchangeably to describe this web search paradigm.



Source