A web search engine crawls the web to find web pages, indexed them, and presents a list of pages matching the search query. There are two types of search engines. One only searches titles and descriptions of sites, and does not search individual pages. The other one is full-text search engine, which indexes billions of web pages and search them by title or content. 

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Many search engines, such as Magellan, Excite, Infoseek, Inktomi, Northern Light, AltaVista and Yahoo!, come and go. Today there are only two popular search engines, Google and Bing. Both are full-text search engine. According to comScore (08/2013), Google market share is 67% and Bing (used by Microsoft and Yahoo) is 29.2%. has 2.7% and AOL 1.2%. Therefore, this site focuses on Google search.

Google runs on a huge distributed network of computers. It has 3 major parts.
  • Googlebot, a web crawler that crawls and saves web pages.
  • Google indexer that reads every word on every page and stores page index information in a huge database.
  • Google query processor that processes your search query, compares with page index and recommends the webpages that are most relevant to your query.