| A web search engine is a software programme designed to search for data on the Internet. The search results are normally called hits and are presented in the form of a list. The information may consist of web pages, images, data and other types of files. Some search tools also gather information available in databases or open directories. Unlike Web directories that are maintained by human editors, search tools work automatically or are a mix of human and algorithmic input.Web search engines function by storing data about countless web pages which they retrieve from the WWW. These pages are retrieved by a web crawler, also known as a spider. It is an automatically-controlled Web browser which follows every link it discovers. The content of each page is then analyzed to determine how to index it. Words, for instance, are extracted from titles, headings or special fields called meta tags. Data about web pages are saved and stored in an index database for further use in queries. Some search engines, such as Google, store the whole or part of the source page (referred to as a cache) and information about web pages, while others, such as AltaVista, store every word of every page they have found. The cached page always comprises the initial search text, since it is the one that was actually indexed. Thus, it can be very helpful because it comprises data that may no longer be available elsewhere.Once a user has typed search words in the search field, the software programme checks its database and shows a listing of the most suitable web pages in accordance with its criteria, normally with a brief summary coupled with the title of the document and sometimes extracts from the text. Some search tools provide an advanced feature called proximity search which allows users to define the distance between key words.The usefulness of a search engine hinges on the relevance of the results it provides. Since there may be millions of web pages that include a particular key word or phrase, web pages can be grouped into relevant and irrelevant ones. Most search engines apply techniques to grade the results to list the "best" results first.How a search engine decides which pages are the best matches, and in what arrangement the results should be shown, is search engine-specific. The methods also change in time, because the use of Internet services undergoes alterations and new techniques emerge. |
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