When a user visits a search engine and enter a query, usually by entering a keyword, search engines index and provides a list of web pages that best matches the criteria, usually accompanied by a brief summary of the document title and sometimes some of the text.
There are other types of search engines: real-time search engine, such as Orase. Machines like this do not use indexes. Machinery necessary information is only collected if there is a new search. When compared with the index-based system used engines like Google, real-time system is superior in several respects: information is always up to date, (almost) no dead links, and fewer system resources required. (Google uses nearly 100,000 computers, Orase only one.) But there are also disadvantages: the search for longer completion.
Benefits depend on the relevance of search engine results it gives. Although there may be millions of web pages containing a word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines use different methods to rank the results to provide "best" results first. The way the machine determines which pages best match, and the order of the pages were not disclosed, is very varied. The methods also change over time as Internet usage changes and new techniques evolve.
Most web search engines are commercial ventures supported by advertising revenue and therefore the most controversial practice, which allows advertisers to pay so that their pages are ranked higher in search results.
Source :http://id.wikipedia.org/wiki/Mesin_pencari
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