Web search engines work by storing information about many web pages, which taken directly from the html itself. These pages are retrieved by a web crawler (also known as a spider) automated web browser which follows every link on the site. Exceptions can be made by using robots.txt. The contents of each page is then analyzed to determine what should be indexed (for example, the words taken from the title, headings, or special fields called meta tags). Data about web pages are stored in an index database for use in later queries.Some search engines such as Google, store all or part of the page source (called cache) as well as information on the web page itself. This cached page always holds the actual search text since it is the one that was actually indexed, so it can be very useful when the content of the current page has been updated and the search terms are no longer in it. This problem might be considered to be a mild form of linkrot, and Google's handling of it increases usability by satisfying user expectations that the search terms will be on the returned webpage. This satisfies the principle of least astonishment since the user normally expects the search terms to be on the returned pages. Increased search relevance makes these cached pages very useful, even beyond the fact that they may contain data that may no longer be available elsewhere.
When a user enters a query into a search engine (typically by using keywords), the engine examines its index and provides a listing of best-matching web pages according to its criteria, usually with a short summary containing the document's title and sometimes parts of the text. Most search engines support the use of the boolean operators AND, OR and NOT to further specify the search query. Some search engines provide an advanced feature called proximity search which allows users to define the distance between keywords.
There are other search engines: search engine real-time, such as Orase. Machines like this do not use indexes. Machinery necessary information is collected only if there is a new search. When compared with systems that use index-based machines such as 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.
The usefulness of a search engine depends on the relevance of the result set it gives back. While there may be millions of web pages that include a particular word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines employ methods to rank the results to provide the "best" results first. How a search engine decides which pages are the best matches, and what order the results should be shown in, varies widely from one engine to another. 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 most controversial practice, which allows advertisers to pay for their pages ranked higher in search results. Those search engines which do not accept money for their search engine results make money by running search related ads alongside the regular search engine results. The search engines make money every time someone clicks on one of these ads.
When a user enters a query into a search engine (typically by using keywords), the engine examines its index and provides a listing of best-matching web pages according to its criteria, usually with a short summary containing the document's title and sometimes parts of the text. Most search engines support the use of the boolean operators AND, OR and NOT to further specify the search query. Some search engines provide an advanced feature called proximity search which allows users to define the distance between keywords.
There are other search engines: search engine real-time, such as Orase. Machines like this do not use indexes. Machinery necessary information is collected only if there is a new search. When compared with systems that use index-based machines such as 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.
The usefulness of a search engine depends on the relevance of the result set it gives back. While there may be millions of web pages that include a particular word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines employ methods to rank the results to provide the "best" results first. How a search engine decides which pages are the best matches, and what order the results should be shown in, varies widely from one engine to another. 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 most controversial practice, which allows advertisers to pay for their pages ranked higher in search results. Those search engines which do not accept money for their search engine results make money by running search related ads alongside the regular search engine results. The search engines make money every time someone clicks on one of these ads.
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