What is a web search engine?
A web search engine is an interactive tool that enables users to locate information available via the World Wide Web. Search engines provide "fill-out" forms and other interfaces so the user can type in a query, submit the request, and retrieve a list of resources that match the search criteria. The hypertext environment makes it possible to offer a link directly from the list of results to the resources themselves.
A single search engine cannot cover every available web resource, but many do contain references to millions of resources. Results may vary from one search engine to the next.