• A Web search engine is a search engine planned to search for information on the World Wide Web. Information may comprise of web pages, images and other kinds of files. Some search engines also extract data available in news books, databases or open directories. Contrasting Web directories, which are retained by human editors, search engines function algorithmically or are a mixture of algorithmic and human input.

    A search engine functions in the following order:
  • Web crawling
  • Indexing
  • Searching

Web search engines work by accumulating information about many web pages, which they repossess from the WWW itself. These pages are recovered by a Web crawler (also known as a spider) — an automated Web browser which chases every link it sees. Elimination can be made by the use of robots.txt. The contents of each page are then evaluated to determine how it should be indexed. Data about web pages are accumulated in an index database for use in later queries. Some search engines such as Google, accumulate all or part of the source page (referred as a cache) as well as information about the web pages, whereas others such as AltaVista accumulate every word of every page they find. This cached page always embraces the actual search text since it is the one that was actually indexed, so it can be very valuable when the content of the current page has been updated and the search terms are no longer in it.
When a user enters a query into a search engine (usually by using key words), the engine inspects its index and provides a listing of best-matching web pages according to its criteria. Most search engines abide the use of the Boolean operators AND, NOT and OR to further specify the search query. Some search engines offer an advanced feature called proximity search which allows users to define the distance between keywords.
The effectiveness of a search engine depends on the relevance of the result set it gives back. While there may be numerous WebPages that include a particular word or phrase, a few pages may be more relevant, popular or authentic than others. Most search engines use methods to rank the results to provide the "best" results first.